diff --git a/pystac/__init__.py b/pystac/__init__.py
index c029ac28..c2a5b53b 100644
--- a/pystac/__init__.py
+++ b/pystac/__init__.py
@@ -33,6 +33,7 @@
     "RangeSummary",
     "Item",
     "Asset",
+    "ItemAssetDefinition",
     "ItemCollection",
     "Provider",
     "ProviderRole",
@@ -81,6 +82,7 @@
 from pystac.summaries import RangeSummary, Summaries
 from pystac.asset import Asset
 from pystac.item import Item
+from pystac.item_assets import ItemAssetDefinition
 from pystac.item_collection import ItemCollection
 from pystac.provider import ProviderRole, Provider
 from pystac.utils import HREF
diff --git a/pystac/collection.py b/pystac/collection.py
index 9c0c24b7..5323b82c 100644
--- a/pystac/collection.py
+++ b/pystac/collection.py
@@ -20,6 +20,7 @@
 from pystac.asset import Asset, Assets
 from pystac.catalog import Catalog
 from pystac.errors import DeprecatedWarning, ExtensionNotImplemented, STACTypeError
+from pystac.item_assets import ItemAssetDefinition, _ItemAssets
 from pystac.layout import HrefLayoutStrategy
 from pystac.link import Link
 from pystac.provider import Provider
@@ -553,6 +554,7 @@ def __init__(
         self.keywords = keywords
         self.providers = providers
         self.summaries = summaries or Summaries.empty()
+        self._item_assets: _ItemAssets | None = None
 
         self.assets = {}
         if assets is not None:
@@ -731,6 +733,27 @@ def get_item(self, id: str, recursive: bool = False) -> Item | None:
                 return super().get_item(id, recursive=recursive)
             raise e
 
+    @property
+    def item_assets(self) -> dict[str, ItemAssetDefinition] | None:
+        if self._item_assets is None and "item_assets" in self.extra_fields:
+            self._item_assets = _ItemAssets(self)
+        return self._item_assets
+
+    @item_assets.setter
+    def item_assets(
+        self, item_assets: dict[str, ItemAssetDefinition | dict[str, Any]] | None
+    ) -> None:
+        # clear out the cached value
+        self._item_assets = None
+
+        if item_assets is None:
+            self.extra_fields.pop("item_assets")
+        else:
+            self.extra_fields["item_assets"] = {
+                k: v if isinstance(v, dict) else v.to_dict()
+                for k, v in item_assets.items()
+            }
+
     def update_extent_from_items(self) -> None:
         """
         Update datetime and bbox based on all items to a single bbox and time window.
diff --git a/pystac/extensions/base.py b/pystac/extensions/base.py
index 1b3016fe..241c8381 100644
--- a/pystac/extensions/base.py
+++ b/pystac/extensions/base.py
@@ -5,7 +5,6 @@
 from abc import ABC, abstractmethod
 from collections.abc import Iterable
 from typing import (
-    TYPE_CHECKING,
     Any,
     Generic,
     TypeVar,
@@ -14,9 +13,6 @@
 
 import pystac
 
-if TYPE_CHECKING:
-    from pystac.extensions.item_assets import AssetDefinition
-
 VERSION_REGEX = re.compile("/v[0-9].[0-9].*/")
 
 
@@ -158,7 +154,7 @@ def has_extension(cls, obj: S) -> bool:
     @classmethod
     def validate_owner_has_extension(
         cls,
-        asset: pystac.Asset | AssetDefinition,
+        asset: pystac.Asset | pystac.ItemAssetDefinition,
         add_if_missing: bool = False,
     ) -> None:
         """
@@ -190,7 +186,7 @@ def validate_owner_has_extension(
     @classmethod
     def ensure_owner_has_extension(
         cls,
-        asset_or_link: pystac.Asset | AssetDefinition | pystac.Link,
+        asset_or_link: pystac.Asset | pystac.ItemAssetDefinition | pystac.Link,
         add_if_missing: bool = False,
     ) -> None:
         """Given an :class:`~pystac.Asset`, checks if the asset's owner has this
diff --git a/pystac/extensions/classification.py b/pystac/extensions/classification.py
index bd8b4557..beb30bbe 100644
--- a/pystac/extensions/classification.py
+++ b/pystac/extensions/classification.py
@@ -16,7 +16,6 @@
 )
 
 import pystac
-from pystac.extensions import item_assets
 from pystac.extensions.base import (
     ExtensionManagementMixin,
     PropertiesExtension,
@@ -27,7 +26,7 @@
 from pystac.serialization.identify import STACJSONDescription, STACVersionID
 from pystac.utils import get_required, map_opt
 
-T = TypeVar("T", pystac.Item, pystac.Asset, item_assets.AssetDefinition, RasterBand)
+T = TypeVar("T", pystac.Item, pystac.Asset, pystac.ItemAssetDefinition, RasterBand)
 
 SCHEMA_URI_PATTERN: str = (
     "https://stac-extensions.github.io/classification/v{version}/schema.json"
@@ -492,7 +491,7 @@ class ClassificationExtension(
     """An abstract class that can be used to extend the properties of
     :class:`~pystac.Item`, :class:`~pystac.Asset`,
     :class:`~pystac.extension.raster.RasterBand`, or
-    :class:`~pystac.extension.item_assets.AssetDefinition` with properties from the
+    :class:`~pystac.ItemAssetDefinition` with properties from the
     :stac-ext:`Classification Extension <classification>`.  This class is generic
     over the type of STAC object being extended.
 
@@ -600,7 +599,7 @@ def ext(cls, obj: T, add_if_missing: bool = False) -> ClassificationExtension[T]
 
         This extension can be applied to instances of :class:`~pystac.Item`,
         :class:`~pystac.Asset`,
-        :class:`~pystac.extensions.item_assets.AssetDefinition`, or
+        :class:`~pystac.ItemAssetDefinition`, or
         :class:`~pystac.extension.raster.RasterBand`.
 
         Raises:
@@ -612,7 +611,7 @@ def ext(cls, obj: T, add_if_missing: bool = False) -> ClassificationExtension[T]
         elif isinstance(obj, pystac.Asset):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(ClassificationExtension[T], AssetClassificationExtension(obj))
-        elif isinstance(obj, item_assets.AssetDefinition):
+        elif isinstance(obj, pystac.ItemAssetDefinition):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(
                 ClassificationExtension[T], ItemAssetsClassificationExtension(obj)
@@ -663,17 +662,19 @@ def __repr__(self) -> str:
 
 
 class ItemAssetsClassificationExtension(
-    ClassificationExtension[item_assets.AssetDefinition]
+    ClassificationExtension[pystac.ItemAssetDefinition]
 ):
     properties: dict[str, Any]
-    asset_defn: item_assets.AssetDefinition
+    asset_defn: pystac.ItemAssetDefinition
 
-    def __init__(self, item_asset: item_assets.AssetDefinition):
+    def __init__(self, item_asset: pystac.ItemAssetDefinition):
         self.asset_defn = item_asset
         self.properties = item_asset.properties
 
     def __repr__(self) -> str:
-        return f"<ItemAssetsClassificationExtension AssetDefinition={self.asset_defn}"
+        return (
+            f"<ItemAssetsClassificationExtension ItemAssetDefinition={self.asset_defn}"
+        )
 
 
 class RasterBandClassificationExtension(ClassificationExtension[RasterBand]):
diff --git a/pystac/extensions/datacube.py b/pystac/extensions/datacube.py
index f21e5b8f..a2eecc0c 100644
--- a/pystac/extensions/datacube.py
+++ b/pystac/extensions/datacube.py
@@ -6,13 +6,12 @@
 from typing import Any, Generic, Literal, TypeVar, Union, cast
 
 import pystac
-from pystac.extensions import item_assets
 from pystac.extensions.base import ExtensionManagementMixin, PropertiesExtension
 from pystac.extensions.hooks import ExtensionHooks
 from pystac.utils import StringEnum, get_required, map_opt
 
 T = TypeVar(
-    "T", pystac.Collection, pystac.Item, pystac.Asset, item_assets.AssetDefinition
+    "T", pystac.Collection, pystac.Item, pystac.Asset, pystac.ItemAssetDefinition
 )
 
 SCHEMA_URI = "https://stac-extensions.github.io/datacube/v2.2.0/schema.json"
@@ -619,7 +618,7 @@ def ext(cls, obj: T, add_if_missing: bool = False) -> DatacubeExtension[T]:
         elif isinstance(obj, pystac.Asset):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(DatacubeExtension[T], AssetDatacubeExtension(obj))
-        elif isinstance(obj, item_assets.AssetDefinition):
+        elif isinstance(obj, pystac.ItemAssetDefinition):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(DatacubeExtension[T], ItemAssetsDatacubeExtension(obj))
         else:
@@ -691,11 +690,11 @@ def __repr__(self) -> str:
         return f"<AssetDatacubeExtension Item id={self.asset_href}>"
 
 
-class ItemAssetsDatacubeExtension(DatacubeExtension[item_assets.AssetDefinition]):
+class ItemAssetsDatacubeExtension(DatacubeExtension[pystac.ItemAssetDefinition]):
     properties: dict[str, Any]
-    asset_defn: item_assets.AssetDefinition
+    asset_defn: pystac.ItemAssetDefinition
 
-    def __init__(self, item_asset: item_assets.AssetDefinition):
+    def __init__(self, item_asset: pystac.ItemAssetDefinition):
         self.asset_defn = item_asset
         self.properties = item_asset.properties
 
diff --git a/pystac/extensions/eo.py b/pystac/extensions/eo.py
index 54b6cbcb..c9628a3c 100644
--- a/pystac/extensions/eo.py
+++ b/pystac/extensions/eo.py
@@ -14,7 +14,7 @@
 )
 
 import pystac
-from pystac.extensions import item_assets, projection, view
+from pystac.extensions import projection, view
 from pystac.extensions.base import (
     ExtensionManagementMixin,
     PropertiesExtension,
@@ -25,7 +25,7 @@
 from pystac.summaries import RangeSummary
 from pystac.utils import get_required, map_opt
 
-T = TypeVar("T", pystac.Item, pystac.Asset, item_assets.AssetDefinition)
+T = TypeVar("T", pystac.Item, pystac.Asset, pystac.ItemAssetDefinition)
 
 SCHEMA_URI: str = "https://stac-extensions.github.io/eo/v1.1.0/schema.json"
 SCHEMA_URIS: list[str] = [
@@ -409,7 +409,7 @@ def ext(cls, obj: T, add_if_missing: bool = False) -> EOExtension[T]:
         elif isinstance(obj, pystac.Asset):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(EOExtension[T], AssetEOExtension(obj))
-        elif isinstance(obj, item_assets.AssetDefinition):
+        elif isinstance(obj, pystac.ItemAssetDefinition):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(EOExtension[T], ItemAssetsEOExtension(obj))
         else:
@@ -536,9 +536,9 @@ def __repr__(self) -> str:
         return f"<AssetEOExtension Asset href={self.asset_href}>"
 
 
-class ItemAssetsEOExtension(EOExtension[item_assets.AssetDefinition]):
+class ItemAssetsEOExtension(EOExtension[pystac.ItemAssetDefinition]):
     properties: dict[str, Any]
-    asset_defn: item_assets.AssetDefinition
+    asset_defn: pystac.ItemAssetDefinition
 
     def _get_bands(self) -> list[Band] | None:
         if BANDS_PROP not in self.properties:
@@ -550,7 +550,7 @@ def _get_bands(self) -> list[Band] | None:
             )
         )
 
-    def __init__(self, item_asset: item_assets.AssetDefinition):
+    def __init__(self, item_asset: pystac.ItemAssetDefinition):
         self.asset_defn = item_asset
         self.properties = item_asset.properties
 
diff --git a/pystac/extensions/ext.py b/pystac/extensions/ext.py
index 2488b1c4..581886c1 100644
--- a/pystac/extensions/ext.py
+++ b/pystac/extensions/ext.py
@@ -1,13 +1,21 @@
 from dataclasses import dataclass
 from typing import Any, Generic, Literal, TypeVar, cast
 
-from pystac import Asset, Catalog, Collection, Item, Link, STACError
+from pystac import (
+    Asset,
+    Catalog,
+    Collection,
+    Item,
+    ItemAssetDefinition,
+    Link,
+    STACError,
+)
 from pystac.extensions.classification import ClassificationExtension
 from pystac.extensions.datacube import DatacubeExtension
 from pystac.extensions.eo import EOExtension
 from pystac.extensions.file import FileExtension
 from pystac.extensions.grid import GridExtension
-from pystac.extensions.item_assets import AssetDefinition, ItemAssetsExtension
+from pystac.extensions.item_assets import ItemAssetsExtension
 from pystac.extensions.mgrs import MgrsExtension
 from pystac.extensions.pointcloud import PointcloudExtension
 from pystac.extensions.projection import ProjectionExtension
@@ -22,8 +30,8 @@
 from pystac.extensions.view import ViewExtension
 from pystac.extensions.xarray_assets import XarrayAssetsExtension
 
-T = TypeVar("T", Asset, AssetDefinition, Link)
-U = TypeVar("U", Asset, AssetDefinition)
+T = TypeVar("T", Asset, ItemAssetDefinition, Link)
+U = TypeVar("U", Asset, ItemAssetDefinition)
 
 EXTENSION_NAMES = Literal[
     "classification",
@@ -107,7 +115,7 @@ def cube(self) -> DatacubeExtension[Collection]:
         return DatacubeExtension.ext(self.stac_object)
 
     @property
-    def item_assets(self) -> dict[str, AssetDefinition]:
+    def item_assets(self) -> dict[str, ItemAssetDefinition]:
         return ItemAssetsExtension.ext(self.stac_object).item_assets
 
     @property
@@ -300,8 +308,8 @@ def xarray(self) -> XarrayAssetsExtension[Asset]:
 
 
 @dataclass
-class ItemAssetExt(_AssetExt[AssetDefinition]):
-    stac_object: AssetDefinition
+class ItemAssetExt(_AssetExt[ItemAssetDefinition]):
+    stac_object: ItemAssetDefinition
 
 
 @dataclass
diff --git a/pystac/extensions/item_assets.py b/pystac/extensions/item_assets.py
index 37c3b72d..68ddff7f 100644
--- a/pystac/extensions/item_assets.py
+++ b/pystac/extensions/item_assets.py
@@ -2,223 +2,24 @@
 
 from __future__ import annotations
 
-from copy import deepcopy
 from typing import TYPE_CHECKING, Any, Literal
 
 import pystac
 from pystac.extensions.base import ExtensionManagementMixin
 from pystac.extensions.hooks import ExtensionHooks
+from pystac.item_assets import (
+    ItemAssetDefinition as AssetDefinition,
+)
 from pystac.serialization.identify import STACJSONDescription, STACVersionID
 from pystac.utils import get_required
 
 if TYPE_CHECKING:
-    from pystac.extensions.ext import ItemAssetExt
+    pass
 
 SCHEMA_URI = "https://stac-extensions.github.io/item-assets/v1.0.0/schema.json"
 
 ITEM_ASSETS_PROP = "item_assets"
 
-ASSET_TITLE_PROP = "title"
-ASSET_DESC_PROP = "description"
-ASSET_TYPE_PROP = "type"
-ASSET_ROLES_PROP = "roles"
-
-
-class AssetDefinition:
-    """Object that contains details about the datafiles that will be included in member
-    Items for this Collection.
-
-    See the :stac-ext:`Asset Object <item-assets#asset-object>` for details.
-    """
-
-    properties: dict[str, Any]
-
-    owner: pystac.Collection | None
-
-    def __init__(
-        self, properties: dict[str, Any], owner: pystac.Collection | None = None
-    ) -> None:
-        self.properties = properties
-        self.owner = owner
-
-    def __eq__(self, o: object) -> bool:
-        if not isinstance(o, AssetDefinition):
-            return NotImplemented
-        return self.to_dict() == o.to_dict()
-
-    @classmethod
-    def create(
-        cls,
-        title: str | None,
-        description: str | None,
-        media_type: str | None,
-        roles: list[str] | None,
-        extra_fields: dict[str, Any] | None = None,
-    ) -> AssetDefinition:
-        """
-        Creates a new asset definition.
-
-        Args:
-            title : Displayed title for clients and users.
-            description : Description of the Asset providing additional details,
-                such as how it was processed or created.
-                `CommonMark 0.29 <http://commonmark.org/>`__ syntax MAY be used
-                for rich text representation.
-            media_type : `media type\
-                <https://github.com/radiantearth/stac-spec/tree/v1.0.0/catalog-spec/catalog-spec.md#media-types>`__
-                 of the asset.
-            roles : `semantic roles
-                <https://github.com/radiantearth/stac-spec/tree/v1.0.0/item-spec/item-spec.md#asset-role-types>`__
-                of the asset, similar to the use of rel in links.
-            extra_fields : Additional fields on the asset definition, e.g. from
-                extensions.
-        """
-        asset_defn = cls({})
-        asset_defn.apply(
-            title=title,
-            description=description,
-            media_type=media_type,
-            roles=roles,
-            extra_fields=extra_fields,
-        )
-        return asset_defn
-
-    def apply(
-        self,
-        title: str | None,
-        description: str | None,
-        media_type: str | None,
-        roles: list[str] | None,
-        extra_fields: dict[str, Any] | None = None,
-    ) -> None:
-        """
-        Sets the properties for this asset definition.
-
-        Args:
-            title : Displayed title for clients and users.
-            description : Description of the Asset providing additional details,
-                such as how it was processed or created.
-                `CommonMark 0.29 <http://commonmark.org/>`__ syntax MAY be used
-                for rich text representation.
-            media_type : `media type\
-                <https://github.com/radiantearth/stac-spec/tree/v1.0.0/catalog-spec/catalog-spec.md#media-types>`__
-                 of the asset.
-            roles : `semantic roles
-                <https://github.com/radiantearth/stac-spec/tree/v1.0.0/item-spec/item-spec.md#asset-role-types>`__
-                of the asset, similar to the use of rel in links.
-            extra_fields : Additional fields on the asset definition, e.g. from
-                extensions.
-        """
-        if extra_fields:
-            self.properties.update(extra_fields)
-        self.title = title
-        self.description = description
-        self.media_type = media_type
-        self.roles = roles
-        self.owner = None
-
-    def set_owner(self, obj: pystac.Collection) -> None:
-        """Sets the owning item of this AssetDefinition.
-
-        The owning item will be used to resolve relative HREFs of this asset.
-
-        Args:
-            obj: The Collection that owns this asset.
-        """
-        self.owner = obj
-
-    @property
-    def title(self) -> str | None:
-        """Gets or sets the displayed title for clients and users."""
-        return self.properties.get(ASSET_TITLE_PROP)
-
-    @title.setter
-    def title(self, v: str | None) -> None:
-        if v is None:
-            self.properties.pop(ASSET_TITLE_PROP, None)
-        else:
-            self.properties[ASSET_TITLE_PROP] = v
-
-    @property
-    def description(self) -> str | None:
-        """Gets or sets a description of the Asset providing additional details, such as
-        how it was processed or created. `CommonMark 0.29 <http://commonmark.org/>`__
-        syntax MAY be used for rich text representation."""
-        return self.properties.get(ASSET_DESC_PROP)
-
-    @description.setter
-    def description(self, v: str | None) -> None:
-        if v is None:
-            self.properties.pop(ASSET_DESC_PROP, None)
-        else:
-            self.properties[ASSET_DESC_PROP] = v
-
-    @property
-    def media_type(self) -> str | None:
-        """Gets or sets the `media type
-        <https://github.com/radiantearth/stac-spec/tree/v1.0.0/catalog-spec/catalog-spec.md#media-types>`__
-        of the asset."""
-        return self.properties.get(ASSET_TYPE_PROP)
-
-    @media_type.setter
-    def media_type(self, v: str | None) -> None:
-        if v is None:
-            self.properties.pop(ASSET_TYPE_PROP, None)
-        else:
-            self.properties[ASSET_TYPE_PROP] = v
-
-    @property
-    def roles(self) -> list[str] | None:
-        """Gets or sets the `semantic roles
-        <https://github.com/radiantearth/stac-spec/tree/v1.0.0/item-spec/item-spec.md#asset-role-types>`__
-        of the asset, similar to the use of rel in links."""
-        return self.properties.get(ASSET_ROLES_PROP)
-
-    @roles.setter
-    def roles(self, v: list[str] | None) -> None:
-        if v is None:
-            self.properties.pop(ASSET_ROLES_PROP, None)
-        else:
-            self.properties[ASSET_ROLES_PROP] = v
-
-    def to_dict(self) -> dict[str, Any]:
-        """Returns a dictionary representing this ``AssetDefinition``."""
-        return deepcopy(self.properties)
-
-    def create_asset(self, href: str) -> pystac.Asset:
-        """Creates a new :class:`~pystac.Asset` instance using the fields from this
-        ``AssetDefinition`` and the given ``href``."""
-        return pystac.Asset(
-            href=href,
-            title=self.title,
-            description=self.description,
-            media_type=self.media_type,
-            roles=self.roles,
-            extra_fields={
-                k: v
-                for k, v in self.properties.items()
-                if k
-                not in {
-                    ASSET_TITLE_PROP,
-                    ASSET_DESC_PROP,
-                    ASSET_TYPE_PROP,
-                    ASSET_ROLES_PROP,
-                }
-            },
-        )
-
-    @property
-    def ext(self) -> ItemAssetExt:
-        """Accessor for extension classes on this item_asset
-
-        Example::
-
-            collection.ext.item_assets["data"].ext.proj.epsg = 4326
-        """
-        from pystac.extensions.ext import ItemAssetExt
-
-        return ItemAssetExt(stac_object=self)
-
 
 class ItemAssetsExtension(ExtensionManagementMixin[pystac.Collection]):
     name: Literal["item_assets"] = "item_assets"
diff --git a/pystac/extensions/pointcloud.py b/pystac/extensions/pointcloud.py
index b82566e7..804c5537 100644
--- a/pystac/extensions/pointcloud.py
+++ b/pystac/extensions/pointcloud.py
@@ -13,7 +13,6 @@
 )
 
 import pystac
-from pystac.extensions import item_assets
 from pystac.extensions.base import (
     ExtensionManagementMixin,
     PropertiesExtension,
@@ -23,7 +22,7 @@
 from pystac.summaries import RangeSummary
 from pystac.utils import StringEnum, get_required, map_opt
 
-T = TypeVar("T", pystac.Item, pystac.Asset, item_assets.AssetDefinition)
+T = TypeVar("T", pystac.Item, pystac.Asset, pystac.ItemAssetDefinition)
 
 SCHEMA_URI: str = "https://stac-extensions.github.io/pointcloud/v1.0.0/schema.json"
 PREFIX: str = "pc:"
@@ -468,7 +467,7 @@ def ext(cls, obj: T, add_if_missing: bool = False) -> PointcloudExtension[T]:
                 )
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(PointcloudExtension[T], AssetPointcloudExtension(obj))
-        elif isinstance(obj, item_assets.AssetDefinition):
+        elif isinstance(obj, pystac.ItemAssetDefinition):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(PointcloudExtension[T], ItemAssetsPointcloudExtension(obj))
         else:
@@ -534,11 +533,11 @@ def __repr__(self) -> str:
         return f"<AssetPointcloudExtension Asset {self.repr_id}>"
 
 
-class ItemAssetsPointcloudExtension(PointcloudExtension[item_assets.AssetDefinition]):
+class ItemAssetsPointcloudExtension(PointcloudExtension[pystac.ItemAssetDefinition]):
     properties: dict[str, Any]
-    asset_defn: item_assets.AssetDefinition
+    asset_defn: pystac.ItemAssetDefinition
 
-    def __init__(self, item_asset: item_assets.AssetDefinition):
+    def __init__(self, item_asset: pystac.ItemAssetDefinition):
         self.asset_defn = item_asset
         self.properties = item_asset.properties
 
diff --git a/pystac/extensions/projection.py b/pystac/extensions/projection.py
index 1982e8b5..8d3cb152 100644
--- a/pystac/extensions/projection.py
+++ b/pystac/extensions/projection.py
@@ -15,7 +15,6 @@
 )
 
 import pystac
-from pystac.extensions import item_assets
 from pystac.extensions.base import (
     ExtensionManagementMixin,
     PropertiesExtension,
@@ -23,7 +22,7 @@
 )
 from pystac.extensions.hooks import ExtensionHooks
 
-T = TypeVar("T", pystac.Item, pystac.Asset, item_assets.AssetDefinition)
+T = TypeVar("T", pystac.Item, pystac.Asset, pystac.ItemAssetDefinition)
 
 SCHEMA_URI: str = "https://stac-extensions.github.io/projection/v1.1.0/schema.json"
 SCHEMA_URIS: list[str] = [
@@ -301,7 +300,7 @@ def ext(cls, obj: T, add_if_missing: bool = False) -> ProjectionExtension[T]:
         elif isinstance(obj, pystac.Asset):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(ProjectionExtension[T], AssetProjectionExtension(obj))
-        elif isinstance(obj, item_assets.AssetDefinition):
+        elif isinstance(obj, pystac.ItemAssetDefinition):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(ProjectionExtension[T], ItemAssetsProjectionExtension(obj))
         else:
@@ -368,11 +367,11 @@ def __repr__(self) -> str:
         return f"<AssetProjectionExtension Asset href={self.asset_href}>"
 
 
-class ItemAssetsProjectionExtension(ProjectionExtension[item_assets.AssetDefinition]):
+class ItemAssetsProjectionExtension(ProjectionExtension[pystac.ItemAssetDefinition]):
     properties: dict[str, Any]
-    asset_defn: item_assets.AssetDefinition
+    asset_defn: pystac.ItemAssetDefinition
 
-    def __init__(self, item_asset: item_assets.AssetDefinition):
+    def __init__(self, item_asset: pystac.ItemAssetDefinition):
         self.asset_defn = item_asset
         self.properties = item_asset.properties
 
diff --git a/pystac/extensions/raster.py b/pystac/extensions/raster.py
index ab57d7c3..eb01ee6b 100644
--- a/pystac/extensions/raster.py
+++ b/pystac/extensions/raster.py
@@ -14,7 +14,6 @@
 )
 
 import pystac
-from pystac.extensions import item_assets
 from pystac.extensions.base import (
     ExtensionManagementMixin,
     PropertiesExtension,
@@ -23,7 +22,7 @@
 from pystac.extensions.hooks import ExtensionHooks
 from pystac.utils import StringEnum, get_opt, get_required, map_opt
 
-T = TypeVar("T", pystac.Asset, item_assets.AssetDefinition)
+T = TypeVar("T", pystac.Asset, pystac.ItemAssetDefinition)
 
 SCHEMA_URI = "https://stac-extensions.github.io/raster/v1.1.0/schema.json"
 SCHEMA_URIS = [
@@ -663,7 +662,7 @@ class RasterExtension(
 ):
     """An abstract class that can be used to extend the properties of an
     :class:`~pystac.Item`, :class:`~pystac.Asset`, or
-    :class:`~pystac.extension.item_assets.AssetDefinition` with properties from
+    :class:`~pystac.extension.pystac.ItemAssetDefinition` with properties from
     the :stac-ext:`Raster Extension <raster>`. This class is generic over
     the type of STAC Object to be extended (e.g. :class:`~pystac.Item`,
     :class:`~pystac.Asset`).
@@ -736,7 +735,7 @@ def ext(cls, obj: T, add_if_missing: bool = False) -> RasterExtension[T]:
         if isinstance(obj, pystac.Asset):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(RasterExtension[T], AssetRasterExtension(obj))
-        elif isinstance(obj, item_assets.AssetDefinition):
+        elif isinstance(obj, pystac.ItemAssetDefinition):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(RasterExtension[T], ItemAssetsRasterExtension(obj))
         else:
@@ -771,16 +770,16 @@ def __repr__(self) -> str:
         return f"<AssetRasterExtension Asset href={self.asset_href}>"
 
 
-class ItemAssetsRasterExtension(RasterExtension[item_assets.AssetDefinition]):
-    asset_definition: item_assets.AssetDefinition
-    """A reference to the :class:`~pystac.extensions.item_assets.AssetDefinition`
+class ItemAssetsRasterExtension(RasterExtension[pystac.ItemAssetDefinition]):
+    asset_definition: pystac.ItemAssetDefinition
+    """A reference to the :class:`~pystac.extensions.pystac.ItemAssetDefinition`
     being extended."""
 
     properties: dict[str, Any]
-    """The :class:`~pystac.extensions.item_assets.AssetDefinition` fields, including
+    """The :class:`~pystac.extensions.pystac.ItemAssetDefinition` fields, including
     extension properties."""
 
-    def __init__(self, item_asset: item_assets.AssetDefinition):
+    def __init__(self, item_asset: pystac.ItemAssetDefinition):
         self.properties = item_asset.properties
         self.asset_definition = item_asset
 
diff --git a/pystac/extensions/sar.py b/pystac/extensions/sar.py
index 112c63a9..16127221 100644
--- a/pystac/extensions/sar.py
+++ b/pystac/extensions/sar.py
@@ -13,7 +13,6 @@
 )
 
 import pystac
-from pystac.extensions import item_assets
 from pystac.extensions.base import (
     ExtensionManagementMixin,
     PropertiesExtension,
@@ -24,7 +23,7 @@
 from pystac.summaries import RangeSummary
 from pystac.utils import StringEnum, get_required, map_opt
 
-T = TypeVar("T", pystac.Item, pystac.Asset, item_assets.AssetDefinition)
+T = TypeVar("T", pystac.Item, pystac.Asset, pystac.ItemAssetDefinition)
 
 SCHEMA_URI: str = "https://stac-extensions.github.io/sar/v1.0.0/schema.json"
 PREFIX: str = "sar:"
@@ -332,7 +331,7 @@ def ext(cls, obj: T, add_if_missing: bool = False) -> SarExtension[T]:
                 )
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(SarExtension[T], AssetSarExtension(obj))
-        elif isinstance(obj, item_assets.AssetDefinition):
+        elif isinstance(obj, pystac.ItemAssetDefinition):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(SarExtension[T], ItemAssetsSarExtension(obj))
         else:
@@ -399,11 +398,11 @@ def __repr__(self) -> str:
         return f"<AssetSarExtension Asset href={self.asset_href}>"
 
 
-class ItemAssetsSarExtension(SarExtension[item_assets.AssetDefinition]):
+class ItemAssetsSarExtension(SarExtension[pystac.ItemAssetDefinition]):
     properties: dict[str, Any]
-    asset_defn: item_assets.AssetDefinition
+    asset_defn: pystac.ItemAssetDefinition
 
-    def __init__(self, item_asset: item_assets.AssetDefinition):
+    def __init__(self, item_asset: pystac.ItemAssetDefinition):
         self.asset_defn = item_asset
         self.properties = item_asset.properties
 
diff --git a/pystac/extensions/sat.py b/pystac/extensions/sat.py
index ab5b638f..ed900b46 100644
--- a/pystac/extensions/sat.py
+++ b/pystac/extensions/sat.py
@@ -14,7 +14,6 @@
 )
 
 import pystac
-from pystac.extensions import item_assets
 from pystac.extensions.base import (
     ExtensionManagementMixin,
     PropertiesExtension,
@@ -24,7 +23,7 @@
 from pystac.summaries import RangeSummary
 from pystac.utils import StringEnum, datetime_to_str, map_opt, str_to_datetime
 
-T = TypeVar("T", pystac.Item, pystac.Asset, item_assets.AssetDefinition)
+T = TypeVar("T", pystac.Item, pystac.Asset, pystac.ItemAssetDefinition)
 
 SCHEMA_URI = "https://stac-extensions.github.io/sat/v1.0.0/schema.json"
 
@@ -163,7 +162,7 @@ def ext(cls, obj: T, add_if_missing: bool = False) -> SatExtension[T]:
         elif isinstance(obj, pystac.Asset):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(SatExtension[T], AssetSatExtension(obj))
-        elif isinstance(obj, item_assets.AssetDefinition):
+        elif isinstance(obj, pystac.ItemAssetDefinition):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(SatExtension[T], ItemAssetsSatExtension(obj))
         else:
@@ -232,11 +231,11 @@ def __repr__(self) -> str:
         return f"<AssetSatExtension Asset href={self.asset_href}>"
 
 
-class ItemAssetsSatExtension(SatExtension[item_assets.AssetDefinition]):
+class ItemAssetsSatExtension(SatExtension[pystac.ItemAssetDefinition]):
     properties: dict[str, Any]
-    asset_defn: item_assets.AssetDefinition
+    asset_defn: pystac.ItemAssetDefinition
 
-    def __init__(self, item_asset: item_assets.AssetDefinition):
+    def __init__(self, item_asset: pystac.ItemAssetDefinition):
         self.asset_defn = item_asset
         self.properties = item_asset.properties
 
diff --git a/pystac/extensions/storage.py b/pystac/extensions/storage.py
index 0b0aa0a1..e7155887 100644
--- a/pystac/extensions/storage.py
+++ b/pystac/extensions/storage.py
@@ -16,7 +16,6 @@
 )
 
 import pystac
-from pystac.extensions import item_assets
 from pystac.extensions.base import (
     ExtensionManagementMixin,
     PropertiesExtension,
@@ -25,7 +24,7 @@
 from pystac.extensions.hooks import ExtensionHooks
 from pystac.utils import StringEnum
 
-T = TypeVar("T", pystac.Item, pystac.Asset, item_assets.AssetDefinition)
+T = TypeVar("T", pystac.Item, pystac.Asset, pystac.ItemAssetDefinition)
 
 SCHEMA_URI: str = "https://stac-extensions.github.io/storage/v1.0.0/schema.json"
 PREFIX: str = "storage:"
@@ -154,7 +153,7 @@ def ext(cls, obj: T, add_if_missing: bool = False) -> StorageExtension[T]:
         elif isinstance(obj, pystac.Asset):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(StorageExtension[T], AssetStorageExtension(obj))
-        elif isinstance(obj, item_assets.AssetDefinition):
+        elif isinstance(obj, pystac.ItemAssetDefinition):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(StorageExtension[T], ItemAssetsStorageExtension(obj))
         else:
@@ -221,11 +220,11 @@ def __repr__(self) -> str:
         return f"<AssetStorageExtension Asset href={self.asset_href}>"
 
 
-class ItemAssetsStorageExtension(StorageExtension[item_assets.AssetDefinition]):
+class ItemAssetsStorageExtension(StorageExtension[pystac.ItemAssetDefinition]):
     properties: dict[str, Any]
-    asset_defn: item_assets.AssetDefinition
+    asset_defn: pystac.ItemAssetDefinition
 
-    def __init__(self, item_asset: item_assets.AssetDefinition):
+    def __init__(self, item_asset: pystac.ItemAssetDefinition):
         self.asset_defn = item_asset
         self.properties = item_asset.properties
 
diff --git a/pystac/extensions/table.py b/pystac/extensions/table.py
index 782d741a..55feda51 100644
--- a/pystac/extensions/table.py
+++ b/pystac/extensions/table.py
@@ -5,13 +5,12 @@
 from typing import Any, Generic, Literal, TypeVar, Union, cast
 
 import pystac
-from pystac.extensions import item_assets
 from pystac.extensions.base import ExtensionManagementMixin, PropertiesExtension
 from pystac.extensions.hooks import ExtensionHooks
 from pystac.utils import get_required
 
 T = TypeVar(
-    "T", pystac.Collection, pystac.Item, pystac.Asset, item_assets.AssetDefinition
+    "T", pystac.Collection, pystac.Item, pystac.Asset, pystac.ItemAssetDefinition
 )
 
 SCHEMA_URI = "https://stac-extensions.github.io/table/v1.2.0/schema.json"
@@ -165,7 +164,7 @@ def ext(cls, obj: T, add_if_missing: bool = False) -> TableExtension[T]:
         if isinstance(obj, pystac.Asset):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(TableExtension[T], AssetTableExtension(obj))
-        elif isinstance(obj, item_assets.AssetDefinition):
+        elif isinstance(obj, pystac.ItemAssetDefinition):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(TableExtension[T], ItemAssetsTableExtension(obj))
         else:
@@ -294,11 +293,11 @@ def __repr__(self) -> str:
         return f"<AssetTableExtension Item id={self.asset_href}>"
 
 
-class ItemAssetsTableExtension(TableExtension[item_assets.AssetDefinition]):
+class ItemAssetsTableExtension(TableExtension[pystac.ItemAssetDefinition]):
     properties: dict[str, Any]
-    asset_defn: item_assets.AssetDefinition
+    asset_defn: pystac.ItemAssetDefinition
 
-    def __init__(self, item_asset: item_assets.AssetDefinition):
+    def __init__(self, item_asset: pystac.ItemAssetDefinition):
         self.asset_defn = item_asset
         self.properties = item_asset.properties
 
diff --git a/pystac/extensions/version.py b/pystac/extensions/version.py
index ee394f48..951a7c95 100644
--- a/pystac/extensions/version.py
+++ b/pystac/extensions/version.py
@@ -20,6 +20,7 @@
     Collection,
     ExtensionTypeError,
     Item,
+    ItemAssetDefinition,
     Link,
     MediaType,
     STACObject,
@@ -28,10 +29,9 @@
 from pystac.errors import DeprecatedWarning
 from pystac.extensions.base import ExtensionManagementMixin, PropertiesExtension
 from pystac.extensions.hooks import ExtensionHooks
-from pystac.extensions.item_assets import AssetDefinition
 from pystac.utils import StringEnum, map_opt
 
-T = TypeVar("T", Collection, Item, Catalog, Asset, AssetDefinition)
+T = TypeVar("T", Collection, Item, Catalog, Asset, ItemAssetDefinition)
 U = TypeVar("U", Collection, Item, Catalog)
 
 SCHEMA_URI = "https://stac-extensions.github.io/version/v1.2.0/schema.json"
@@ -395,10 +395,10 @@ def __repr__(self) -> str:
         return f"<AssetVersionExtension Asset href={self.asset.href}>"
 
 
-class ItemAssetsViewExtension(BaseVersionExtension[AssetDefinition]):
+class ItemAssetsViewExtension(BaseVersionExtension[ItemAssetDefinition]):
     properties: dict[str, Any]
 
-    def __init__(self, item_asset: AssetDefinition):
+    def __init__(self, item_asset: ItemAssetDefinition):
         self.properties = item_asset.properties
 
 
diff --git a/pystac/extensions/view.py b/pystac/extensions/view.py
index 3a91249e..29470d87 100644
--- a/pystac/extensions/view.py
+++ b/pystac/extensions/view.py
@@ -6,7 +6,6 @@
 from typing import Any, Generic, Literal, TypeVar, Union, cast
 
 import pystac
-from pystac.extensions import item_assets
 from pystac.extensions.base import (
     ExtensionManagementMixin,
     PropertiesExtension,
@@ -15,7 +14,7 @@
 from pystac.extensions.hooks import ExtensionHooks
 from pystac.summaries import RangeSummary
 
-T = TypeVar("T", pystac.Item, pystac.Asset, item_assets.AssetDefinition)
+T = TypeVar("T", pystac.Item, pystac.Asset, pystac.ItemAssetDefinition)
 
 SCHEMA_URI: str = "https://stac-extensions.github.io/view/v1.0.0/schema.json"
 PREFIX: str = "view:"
@@ -166,7 +165,7 @@ def ext(cls, obj: T, add_if_missing: bool = False) -> ViewExtension[T]:
         elif isinstance(obj, pystac.Asset):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(ViewExtension[T], AssetViewExtension(obj))
-        elif isinstance(obj, item_assets.AssetDefinition):
+        elif isinstance(obj, pystac.ItemAssetDefinition):
             cls.ensure_owner_has_extension(obj, add_if_missing)
             return cast(ViewExtension[T], ItemAssetsViewExtension(obj))
         else:
@@ -233,11 +232,11 @@ def __repr__(self) -> str:
         return f"<AssetViewExtension Asset href={self.asset_href}>"
 
 
-class ItemAssetsViewExtension(ViewExtension[item_assets.AssetDefinition]):
+class ItemAssetsViewExtension(ViewExtension[pystac.ItemAssetDefinition]):
     properties: dict[str, Any]
-    asset_defn: item_assets.AssetDefinition
+    asset_defn: pystac.ItemAssetDefinition
 
-    def __init__(self, item_asset: item_assets.AssetDefinition):
+    def __init__(self, item_asset: pystac.ItemAssetDefinition):
         self.asset_defn = item_asset
         self.properties = item_asset.properties
 
diff --git a/pystac/item_assets.py b/pystac/item_assets.py
new file mode 100644
index 00000000..4bcaec9d
--- /dev/null
+++ b/pystac/item_assets.py
@@ -0,0 +1,243 @@
+"""Implements the ``Item Asset Definition <item-assets>``."""
+
+from __future__ import annotations
+
+from copy import deepcopy
+from typing import TYPE_CHECKING, Any
+
+import pystac
+
+if TYPE_CHECKING:
+    from pystac.extensions.ext import ItemAssetExt
+
+
+ASSET_TITLE_PROP = "title"
+ASSET_DESC_PROP = "description"
+ASSET_TYPE_PROP = "type"
+ASSET_ROLES_PROP = "roles"
+
+
+class ItemAssetDefinition:
+    """Object that contains details about the datafiles that will be included in member
+    Items for this Collection.
+
+    See the `Item Asset Definition Object <item-assets#asset-object>` for details.
+    """
+
+    properties: dict[str, Any]
+
+    owner: pystac.Collection | None
+
+    def __init__(
+        self, properties: dict[str, Any], owner: pystac.Collection | None = None
+    ) -> None:
+        self.properties = properties
+        self.owner = owner
+
+    def __eq__(self, o: object) -> bool:
+        if not isinstance(o, ItemAssetDefinition):
+            return NotImplemented
+        return self.to_dict() == o.to_dict()
+
+    @classmethod
+    def create(
+        cls,
+        title: str | None,
+        description: str | None,
+        media_type: str | None,
+        roles: list[str] | None,
+        extra_fields: dict[str, Any] | None = None,
+    ) -> ItemAssetDefinition:
+        """
+        Creates a new asset definition.
+
+        Args:
+            title : Displayed title for clients and users.
+            description : Description of the Asset providing additional details,
+                such as how it was processed or created.
+                `CommonMark 0.29 <http://commonmark.org/>`__ syntax MAY be used
+                for rich text representation.
+            media_type : `media type\
+                <https://github.com/radiantearth/stac-spec/tree/v1.0.0/catalog-spec/catalog-spec.md#media-types>`__
+                 of the asset.
+            roles : `semantic roles
+                <https://github.com/radiantearth/stac-spec/tree/v1.0.0/item-spec/item-spec.md#asset-role-types>`__
+                of the asset, similar to the use of rel in links.
+            extra_fields : Additional fields on the asset definition, e.g. from
+                extensions.
+        """
+        asset_defn = cls({})
+        asset_defn.apply(
+            title=title,
+            description=description,
+            media_type=media_type,
+            roles=roles,
+            extra_fields=extra_fields,
+        )
+        return asset_defn
+
+    def apply(
+        self,
+        title: str | None,
+        description: str | None,
+        media_type: str | None,
+        roles: list[str] | None,
+        extra_fields: dict[str, Any] | None = None,
+    ) -> None:
+        """
+        Sets the properties for this asset definition.
+
+        Args:
+            title : Displayed title for clients and users.
+            description : Description of the Asset providing additional details,
+                such as how it was processed or created.
+                `CommonMark 0.29 <http://commonmark.org/>`__ syntax MAY be used
+                for rich text representation.
+            media_type : `media type\
+                <https://github.com/radiantearth/stac-spec/tree/v1.0.0/catalog-spec/catalog-spec.md#media-types>`__
+                 of the asset.
+            roles : `semantic roles
+                <https://github.com/radiantearth/stac-spec/tree/v1.0.0/item-spec/item-spec.md#asset-role-types>`__
+                of the asset, similar to the use of rel in links.
+            extra_fields : Additional fields on the asset definition, e.g. from
+                extensions.
+        """
+        if extra_fields:
+            self.properties.update(extra_fields)
+        self.title = title
+        self.description = description
+        self.media_type = media_type
+        self.roles = roles
+        self.owner = None
+
+    def set_owner(self, obj: pystac.Collection) -> None:
+        """Sets the owning item of this ItemAssetDefinition.
+
+        The owning item will be used to resolve relative HREFs of this asset.
+
+        Args:
+            obj: The Collection that owns this asset.
+        """
+        self.owner = obj
+
+    @property
+    def title(self) -> str | None:
+        """Gets or sets the displayed title for clients and users."""
+        return self.properties.get(ASSET_TITLE_PROP)
+
+    @title.setter
+    def title(self, v: str | None) -> None:
+        if v is None:
+            self.properties.pop(ASSET_TITLE_PROP, None)
+        else:
+            self.properties[ASSET_TITLE_PROP] = v
+
+    @property
+    def description(self) -> str | None:
+        """Gets or sets a description of the Asset providing additional details, such as
+        how it was processed or created. `CommonMark 0.29 <http://commonmark.org/>`__
+        syntax MAY be used for rich text representation."""
+        return self.properties.get(ASSET_DESC_PROP)
+
+    @description.setter
+    def description(self, v: str | None) -> None:
+        if v is None:
+            self.properties.pop(ASSET_DESC_PROP, None)
+        else:
+            self.properties[ASSET_DESC_PROP] = v
+
+    @property
+    def media_type(self) -> str | None:
+        """Gets or sets the `media type
+        <https://github.com/radiantearth/stac-spec/tree/v1.0.0/catalog-spec/catalog-spec.md#media-types>`__
+        of the asset."""
+        return self.properties.get(ASSET_TYPE_PROP)
+
+    @media_type.setter
+    def media_type(self, v: str | None) -> None:
+        if v is None:
+            self.properties.pop(ASSET_TYPE_PROP, None)
+        else:
+            self.properties[ASSET_TYPE_PROP] = v
+
+    @property
+    def roles(self) -> list[str] | None:
+        """Gets or sets the `semantic roles
+        <https://github.com/radiantearth/stac-spec/tree/v1.0.0/item-spec/item-spec.md#asset-role-types>`__
+        of the asset, similar to the use of rel in links."""
+        return self.properties.get(ASSET_ROLES_PROP)
+
+    @roles.setter
+    def roles(self, v: list[str] | None) -> None:
+        if v is None:
+            self.properties.pop(ASSET_ROLES_PROP, None)
+        else:
+            self.properties[ASSET_ROLES_PROP] = v
+
+    def to_dict(self) -> dict[str, Any]:
+        """Returns a dictionary representing this ``ItemAssetDefinition``."""
+        return deepcopy(self.properties)
+
+    def create_asset(self, href: str) -> pystac.Asset:
+        """Creates a new :class:`~pystac.Asset` instance using the fields from this
+        ``ItemAssetDefinition`` and the given ``href``."""
+        return pystac.Asset(
+            href=href,
+            title=self.title,
+            description=self.description,
+            media_type=self.media_type,
+            roles=self.roles,
+            extra_fields={
+                k: v
+                for k, v in self.properties.items()
+                if k
+                not in {
+                    ASSET_TITLE_PROP,
+                    ASSET_DESC_PROP,
+                    ASSET_TYPE_PROP,
+                    ASSET_ROLES_PROP,
+                }
+            },
+        )
+
+    @property
+    def ext(self) -> ItemAssetExt:
+        """Accessor for extension classes on this item_asset
+
+        Example::
+
+            collection.item_assets["data"].ext.proj.epsg = 4326
+        """
+        from pystac.extensions.ext import ItemAssetExt
+
+        return ItemAssetExt(stac_object=self)
+
+
+class _ItemAssets(dict):  # type:ignore
+    """Private class for exposing item_assets as a dict
+
+    This class coerces values to ``ItemAssetDefinition``s and
+    sets that owner on all ``ItemAssetDefinition``s to the collection
+    that it is owned by.
+    """
+
+    collection: pystac.Collection
+
+    def __init__(self, collection: pystac.Collection) -> None:
+        self.collection = collection
+        if not collection.extra_fields.get("item_assets"):
+            collection.extra_fields["item_assets"] = {}
+        self.update(collection.extra_fields["item_assets"])
+
+    def __setitem__(self, key: str, value: Any) -> None:
+        if isinstance(value, ItemAssetDefinition):
+            asset_definition = value
+            asset_definition.set_owner(self.collection)
+        else:
+            asset_definition = ItemAssetDefinition(value, self.collection)
+        self.collection.extra_fields["item_assets"][key] = asset_definition.properties
+        super().__setitem__(key, asset_definition)
+
+    def update(self, *args: Any, **kwargs: Any) -> None:
+        for k, v in dict(*args, **kwargs).items():
+            self[k] = v