diff --git a/assembly.go b/assembly.go
index 2681f1d..5f6b5d6 100644
--- a/assembly.go
+++ b/assembly.go
@@ -14,12 +14,16 @@ import (
 type Assembly struct {
 	// NotifiyURL specifies a URL to which a request will be sent once the
 	// assembly finishes.
-	// See https://transloadit.com/docs#notifications.
+	// See https://transloadit.com/docs#notifications
 	NotifyURL string
 	// TemplateID specifies a optional template from which the encoding
 	// instructions will be fetched.
 	// See https://transloadit.com/docs/#15-templates
 	TemplateID string
+	// Fields specifies additional key-value pairs that can be accessed by
+	// Assembly Instructions to allow customizing steps on a per-assembly basis.
+	// See https://transloadit.com/docs/#assembly-variables
+	Fields map[string]interface{}
 
 	steps   map[string]map[string]interface{}
 	readers []*upload
@@ -139,6 +143,7 @@ type FileInfo struct {
 // an assembly using Client.StartAssembly.
 func NewAssembly() Assembly {
 	return Assembly{
+		Fields:  make(map[string]interface{}),
 		steps:   make(map[string]map[string]interface{}),
 		readers: make([]*upload, 0),
 	}
@@ -220,6 +225,10 @@ func (assembly *Assembly) makeRequest(ctx context.Context, client *Client) (*htt
 		options["steps"] = assembly.steps
 	}
 
+	if len(assembly.Fields) != 0 {
+		options["fields"] = assembly.Fields
+	}
+
 	if assembly.TemplateID != "" {
 		options["template_id"] = assembly.TemplateID
 	}
diff --git a/assembly_test.go b/assembly_test.go
index 3328b8c..2cc9e40 100644
--- a/assembly_test.go
+++ b/assembly_test.go
@@ -31,6 +31,8 @@ func TestStartAssembly_Success(t *testing.T) {
 	})
 
 	assembly.NotifyURL = "https://example.com/"
+	assembly.Fields["string_test"] = "foo"
+	assembly.Fields["number_test"] = 100
 
 	info, err := client.StartAssembly(ctx, assembly)
 	if err != nil {
@@ -45,6 +47,15 @@ func TestStartAssembly_Success(t *testing.T) {
 		t.Fatal("wrong notify url")
 	}
 
+	if info.Fields["string_test"] != "foo" {
+		t.Fatal("wrong field string_test")
+	}
+
+	fmt.Printf("%#v\n", info.Fields)
+	if info.Fields["number_test"] != float64(100) {
+		t.Fatal("wrong field number_test")
+	}
+
 	if len(info.Uploads) != 2 {
 		t.Fatal("wrong number of uploads")
 	}