diff --git a/pkg/runtime/extension.go b/pkg/runtime/extension.go
new file mode 100644
index 00000000000..4d731cb4838
--- /dev/null
+++ b/pkg/runtime/extension.go
@@ -0,0 +1,55 @@
+/*
+Copyright 2014 Google Inc. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package runtime
+
+import (
+	"gopkg.in/v1/yaml"
+)
+
+func (re *RawExtension) UnmarshalJSON(in []byte) error {
+	re.RawJSON = in
+	return nil
+}
+
+func (re *RawExtension) MarshalJSON() ([]byte, error) {
+	return re.RawJSON, nil
+}
+
+// SetYAML implements the yaml.Setter interface.
+func (re *RawExtension) SetYAML(tag string, value interface{}) bool {
+	if value == nil {
+		re.RawJSON = []byte("null")
+		return true
+	}
+	// Why does the yaml package send value as a map[interface{}]interface{}?
+	// It's especially frustrating because encoding/json does the right thing
+	// by giving a []byte. So here we do the embarrasing thing of re-encode and
+	// de-encode the right way.
+	// TODO: Write a version of Decode that uses reflect to turn this value
+	// into an API object.
+	b, err := yaml.Marshal(value)
+	if err != nil {
+		panic("yaml can't reverse its own object")
+	}
+	re.RawJSON = b
+	return true
+}
+
+// GetYAML implements the yaml.Getter interface.
+func (re *RawExtension) GetYAML() (tag string, value interface{}) {
+	return tag, re.RawJSON
+}
diff --git a/pkg/runtime/helper_test.go b/pkg/runtime/helper_test.go
deleted file mode 100644
index b68178a3227..00000000000
--- a/pkg/runtime/helper_test.go
+++ /dev/null
@@ -1,81 +0,0 @@
-/*
-Copyright 2014 Google Inc. All rights reserved.
-
-Licensed under the Apache License, Version 2.0 (the "License");
-you may not use this file except in compliance with the License.
-You may obtain a copy of the License at
-
-    http://www.apache.org/licenses/LICENSE-2.0
-
-Unless required by applicable law or agreed to in writing, software
-distributed under the License is distributed on an "AS IS" BASIS,
-WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-See the License for the specific language governing permissions and
-limitations under the License.
-*/
-
-package runtime_test
-
-import (
-	"reflect"
-	"testing"
-
-	"github.com/GoogleCloudPlatform/kubernetes/pkg/api"
-	_ "github.com/GoogleCloudPlatform/kubernetes/pkg/api/v1beta1"
-	"github.com/GoogleCloudPlatform/kubernetes/pkg/runtime"
-)
-
-func TestEncode(t *testing.T) {
-	pod := &api.Pod{
-		Labels: map[string]string{"name": "foo"},
-	}
-	obj := runtime.Object(pod)
-	data, err := runtime.DefaultScheme.Encode(obj)
-	obj2, err2 := runtime.DefaultScheme.Decode(data)
-	if err != nil || err2 != nil {
-		t.Fatalf("Failure: '%v' '%v'", err, err2)
-	}
-	if _, ok := obj2.(*api.Pod); !ok {
-		t.Fatalf("Got wrong type")
-	}
-	if !reflect.DeepEqual(obj2, pod) {
-		t.Errorf("Expected:\n %#v,\n Got:\n %#v", &pod, obj2)
-	}
-}
-
-func TestBadJSONRejection(t *testing.T) {
-	badJSONMissingKind := []byte(`{ }`)
-	if _, err := runtime.DefaultScheme.Decode(badJSONMissingKind); err == nil {
-		t.Errorf("Did not reject despite lack of kind field: %s", badJSONMissingKind)
-	}
-	badJSONUnknownType := []byte(`{"kind": "bar"}`)
-	if _, err1 := runtime.DefaultScheme.Decode(badJSONUnknownType); err1 == nil {
-		t.Errorf("Did not reject despite use of unknown type: %s", badJSONUnknownType)
-	}
-	/*badJSONKindMismatch := []byte(`{"kind": "Pod"}`)
-	if err2 := DecodeInto(badJSONKindMismatch, &Minion{}); err2 == nil {
-		t.Errorf("Kind is set but doesn't match the object type: %s", badJSONKindMismatch)
-	}*/
-}
-
-func TestExtractList(t *testing.T) {
-	pl := &api.PodList{
-		Items: []api.Pod{
-			{JSONBase: api.JSONBase{ID: "1"}},
-			{JSONBase: api.JSONBase{ID: "2"}},
-			{JSONBase: api.JSONBase{ID: "3"}},
-		},
-	}
-	list, err := runtime.ExtractList(pl)
-	if err != nil {
-		t.Fatalf("Unexpected error %v", err)
-	}
-	if e, a := len(list), len(pl.Items); e != a {
-		t.Fatalf("Expected %v, got %v", e, a)
-	}
-	for i := range list {
-		if e, a := list[i].(*api.Pod).ID, pl.Items[i].ID; e != a {
-			t.Fatalf("Expected %v, got %v", e, a)
-		}
-	}
-}
diff --git a/pkg/runtime/helper.go b/pkg/runtime/scheme.go
similarity index 67%
rename from pkg/runtime/helper.go
rename to pkg/runtime/scheme.go
index 840a9fddcca..a90c67da31e 100644
--- a/pkg/runtime/helper.go
+++ b/pkg/runtime/scheme.go
@@ -34,6 +34,149 @@ type Scheme struct {
 	raw *conversion.Scheme
 }
 
+var namedSchemes map[string]*Scheme
+
+// GetScheme returns the scheme with the given name, creating it if necessary.
+// Important: You may not modify the returned *Scheme except from init() functions.
+func GetScheme(schemeName string) *Scheme {
+	if namedSchemes == nil {
+		namedSchemes = map[string]*Scheme{}
+	}
+	if s, ok := namedSchemes[schemeName]; ok {
+		return s
+	}
+	s := NewScheme("", "")
+	namedSchemes[schemeName] = s
+	return s
+}
+
+// fromScope gets the input version, desired output version, and desired Scheme
+// from a conversion.Scope.
+func fromScope(s conversion.Scope) (inVersion, outVersion string, scheme *Scheme) {
+	scheme = DefaultScheme
+	inVersion = s.Meta()["srcVersion"].(string)
+	outVersion = s.Meta()["destVersion"].(string)
+	// If a scheme tag was provided, use it. Look at the struct tag corresponding
+	// to version "".
+	if name := s.SrcTag().Get("scheme"); inVersion == "" && name != "" {
+		scheme = GetScheme(name)
+	}
+	if name := s.DestTag().Get("scheme"); outVersion == "" && name != "" {
+		scheme = GetScheme(name)
+	}
+	return inVersion, outVersion, scheme
+}
+
+func init() {
+	// Set up a generic mapping between RawExtension and EmbeddedObject.
+	DefaultScheme.AddConversionFuncs(
+		embeddedObjectToRawExtension,
+		rawExtensionToEmbeddedObject,
+	)
+}
+
+// emptyPlugin is used to copy the Kind field to and from plugin objects.
+type emptyPlugin struct {
+	PluginBase `json:",inline" yaml:",inline"`
+}
+
+// embeddedObjectToRawExtension does the conversion you would expect from the name, using the information
+// given in conversion.Scope. It's placed in the DefaultScheme as a ConversionFunc to enable plugins;
+// see the comment for RawExtension.
+func embeddedObjectToRawExtension(in *EmbeddedObject, out *RawExtension, s conversion.Scope) error {
+	if in.Object == nil {
+		out.RawJSON = []byte("null")
+		return nil
+	}
+
+	// Figure out the type and kind of the output object.
+	_, outVersion, scheme := fromScope(s)
+	_, kind, err := scheme.raw.ObjectVersionAndKind(in.Object)
+	if err != nil {
+		return err
+	}
+
+	// Manufacture an object of this type and kind.
+	outObj, err := scheme.New(outVersion, kind)
+	if err != nil {
+		return err
+	}
+
+	// Manually do the conversion.
+	err = s.Convert(in.Object, outObj, 0)
+	if err != nil {
+		return err
+	}
+
+	// Copy the kind field into the ouput object.
+	err = s.Convert(
+		&emptyPlugin{PluginBase: PluginBase{Kind: kind}},
+		outObj,
+		conversion.SourceToDest|conversion.IgnoreMissingFields|conversion.AllowDifferentFieldTypeNames,
+	)
+	if err != nil {
+		return err
+	}
+	// Because we provide the correct version, EncodeToVersion will not attempt a conversion.
+	raw, err := scheme.EncodeToVersion(outObj, outVersion)
+	if err != nil {
+		// TODO: if this fails, create an Unknown-- maybe some other
+		// component will understand it.
+		return err
+	}
+	out.RawJSON = raw
+	return nil
+}
+
+// rawExtensionToEmbeddedObject does the conversion you would expect from the name, using the information
+// given in conversion.Scope. It's placed in the DefaultScheme as a ConversionFunc to enable plugins;
+// see the comment for RawExtension.
+func rawExtensionToEmbeddedObject(in *RawExtension, out *EmbeddedObject, s conversion.Scope) error {
+	if len(in.RawJSON) == 4 && string(in.RawJSON) == "null" {
+		out.Object = nil
+		return nil
+	}
+	// Figure out the type and kind of the output object.
+	inVersion, outVersion, scheme := fromScope(s)
+	_, kind, err := scheme.raw.DataVersionAndKind(in.RawJSON)
+	if err != nil {
+		return err
+	}
+
+	// We have to make this object ourselves because we don't store the version field for
+	// plugin objects.
+	inObj, err := scheme.New(inVersion, kind)
+	if err != nil {
+		return err
+	}
+
+	err = scheme.DecodeInto(in.RawJSON, inObj)
+	if err != nil {
+		return err
+	}
+
+	// Make the desired internal version, and do the conversion.
+	outObj, err := scheme.New(outVersion, kind)
+	if err != nil {
+		return err
+	}
+	err = scheme.Convert(inObj, outObj)
+	if err != nil {
+		return err
+	}
+	// Last step, clear the Kind field; that should always be blank in memory.
+	err = s.Convert(
+		&emptyPlugin{PluginBase: PluginBase{Kind: ""}},
+		outObj,
+		conversion.SourceToDest|conversion.IgnoreMissingFields|conversion.AllowDifferentFieldTypeNames,
+	)
+	if err != nil {
+		return err
+	}
+	out.Object = outObj
+	return nil
+}
+
 // NewScheme creates a new Scheme. A default scheme is provided and accessible
 // as the "DefaultScheme" variable.
 func NewScheme(internalVersion, externalVersion string) *Scheme {
@@ -54,6 +197,13 @@ func (s *Scheme) AddKnownTypes(version string, types ...Object) {
 	s.raw.AddKnownTypes(version, interfaces...)
 }
 
+// AddKnownTypeWithName is like AddKnownTypes, but it lets you specify what this type should
+// be encoded as. Useful for testing when you don't want to make multiple packages to define
+// your structs.
+func (s *Scheme) AddKnownTypeWithName(version, kind string, obj Object) {
+	s.raw.AddKnownTypeWithName(version, kind, obj)
+}
+
 // New returns a new API object of the given version ("" for internal
 // representation) and name, or an error if it hasn't been registered.
 func (s *Scheme) New(versionName, typeName string) (Object, error) {
@@ -153,6 +303,11 @@ func (s *Scheme) Encode(obj Object) (data []byte, err error) {
 	return s.raw.Encode(obj)
 }
 
+// EncodeToVersion is like Encode, but lets you specify the destination version.
+func (s *Scheme) EncodeToVersion(obj Object, destVersion string) (data []byte, err error) {
+	return s.raw.EncodeToVersion(obj, destVersion)
+}
+
 // enforcePtr ensures that obj is a pointer of some sort. Returns a reflect.Value of the
 // dereferenced pointer, ensuring that it is settable/addressable.
 // Returns an error if this is not possible.
diff --git a/pkg/runtime/scheme_test.go b/pkg/runtime/scheme_test.go
new file mode 100644
index 00000000000..b44454ba42d
--- /dev/null
+++ b/pkg/runtime/scheme_test.go
@@ -0,0 +1,257 @@
+/*
+Copyright 2014 Google Inc. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package runtime_test
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/GoogleCloudPlatform/kubernetes/pkg/conversion"
+	"github.com/GoogleCloudPlatform/kubernetes/pkg/runtime"
+)
+
+type JSONBase struct {
+	Kind       string `json:"kind,omitempty" yaml:"kind,omitempty"`
+	APIVersion string `json:"apiVersion,omitempty" yaml:"apiVersion,omitempty"`
+}
+
+type InternalSimple struct {
+	JSONBase   `json:",inline" yaml:",inline"`
+	TestString string `json:"testString" yaml:"testString"`
+}
+
+type ExternalSimple struct {
+	JSONBase   `json:",inline" yaml:",inline"`
+	TestString string `json:"testString" yaml:"testString"`
+}
+
+func (*InternalSimple) IsAnAPIObject() {}
+func (*ExternalSimple) IsAnAPIObject() {}
+
+func TestScheme(t *testing.T) {
+	runtime.DefaultScheme.AddKnownTypeWithName("", "Simple", &InternalSimple{})
+	runtime.DefaultScheme.AddKnownTypeWithName("externalVersion", "Simple", &ExternalSimple{})
+
+	internalToExternalCalls := 0
+	externalToInternalCalls := 0
+
+	// Register functions to verify that scope.Meta() gets set correctly.
+	err := runtime.DefaultScheme.AddConversionFuncs(
+		func(in *InternalSimple, out *ExternalSimple, scope conversion.Scope) error {
+			if e, a := "", scope.Meta()["srcVersion"].(string); e != a {
+				t.Errorf("Expected '%v', got '%v'", e, a)
+			}
+			if e, a := "externalVersion", scope.Meta()["destVersion"].(string); e != a {
+				t.Errorf("Expected '%v', got '%v'", e, a)
+			}
+			scope.Convert(&in.JSONBase, &out.JSONBase, 0)
+			scope.Convert(&in.TestString, &out.TestString, 0)
+			internalToExternalCalls++
+			return nil
+		},
+		func(in *ExternalSimple, out *InternalSimple, scope conversion.Scope) error {
+			if e, a := "externalVersion", scope.Meta()["srcVersion"].(string); e != a {
+				t.Errorf("Expected '%v', got '%v'", e, a)
+			}
+			if e, a := "", scope.Meta()["destVersion"].(string); e != a {
+				t.Errorf("Expected '%v', got '%v'", e, a)
+			}
+			scope.Convert(&in.JSONBase, &out.JSONBase, 0)
+			scope.Convert(&in.TestString, &out.TestString, 0)
+			externalToInternalCalls++
+			return nil
+		},
+	)
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+	simple := &InternalSimple{
+		TestString: "foo",
+	}
+
+	// Test Encode, Decode, and DecodeInto
+	obj := runtime.Object(simple)
+	data, err := runtime.DefaultScheme.EncodeToVersion(obj, "externalVersion")
+	obj2, err2 := runtime.DefaultScheme.Decode(data)
+	obj3 := &InternalSimple{}
+	err3 := runtime.DefaultScheme.DecodeInto(data, obj3)
+	if err != nil || err2 != nil {
+		t.Fatalf("Failure: '%v' '%v' '%v'", err, err2, err3)
+	}
+	if _, ok := obj2.(*InternalSimple); !ok {
+		t.Fatalf("Got wrong type")
+	}
+	if e, a := simple, obj2; !reflect.DeepEqual(e, a) {
+		t.Errorf("Expected:\n %#v,\n Got:\n %#v", e, a)
+	}
+	if e, a := simple, obj3; !reflect.DeepEqual(e, a) {
+		t.Errorf("Expected:\n %#v,\n Got:\n %#v", e, a)
+	}
+
+	// Test Convert
+	external := &ExternalSimple{}
+	err = runtime.DefaultScheme.Convert(simple, external)
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+	if e, a := simple.TestString, external.TestString; e != a {
+		t.Errorf("Expected %v, got %v", e, a)
+	}
+
+	// Encode and Convert should each have caused an increment.
+	if e, a := 2, internalToExternalCalls; e != a {
+		t.Errorf("Expected %v, got %v", e, a)
+	}
+	// Decode and DecodeInto should each have caused an increment.
+	if e, a := 2, externalToInternalCalls; e != a {
+		t.Errorf("Expected %v, got %v", e, a)
+	}
+}
+
+func TestBadJSONRejection(t *testing.T) {
+	badJSONMissingKind := []byte(`{ }`)
+	if _, err := runtime.DefaultScheme.Decode(badJSONMissingKind); err == nil {
+		t.Errorf("Did not reject despite lack of kind field: %s", badJSONMissingKind)
+	}
+	badJSONUnknownType := []byte(`{"kind": "bar"}`)
+	if _, err1 := runtime.DefaultScheme.Decode(badJSONUnknownType); err1 == nil {
+		t.Errorf("Did not reject despite use of unknown type: %s", badJSONUnknownType)
+	}
+	/*badJSONKindMismatch := []byte(`{"kind": "Pod"}`)
+	if err2 := DecodeInto(badJSONKindMismatch, &Minion{}); err2 == nil {
+		t.Errorf("Kind is set but doesn't match the object type: %s", badJSONKindMismatch)
+	}*/
+}
+
+type ExtensionA struct {
+	runtime.PluginBase `json:",inline" yaml:",inline"`
+	TestString         string `json:"testString" yaml:"testString"`
+}
+
+type ExtensionB struct {
+	runtime.PluginBase `json:",inline" yaml:",inline"`
+	TestString         string `json:"testString" yaml:"testString"`
+}
+
+type ExternalExtensionType struct {
+	JSONBase  `json:",inline" yaml:",inline"`
+	Extension runtime.RawExtension `json:"extension" yaml:"extension"`
+}
+
+type InternalExtensionType struct {
+	JSONBase  `json:",inline" yaml:",inline"`
+	Extension runtime.EmbeddedObject `json:"extension" yaml:"extension" scheme:"testExtension"`
+}
+
+type InternalExtensionTypeNoScheme struct {
+	JSONBase  `json:",inline" yaml:",inline"`
+	Extension runtime.EmbeddedObject `json:"extension" yaml:"extension"`
+}
+
+func (*ExtensionA) IsAnAPIObject()                    {}
+func (*ExtensionB) IsAnAPIObject()                    {}
+func (*ExternalExtensionType) IsAnAPIObject()         {}
+func (*InternalExtensionType) IsAnAPIObject()         {}
+func (*InternalExtensionTypeNoScheme) IsAnAPIObject() {}
+
+func TestExtensionMappingWithScheme(t *testing.T) {
+	runtime.DefaultScheme.AddKnownTypeWithName("", "ExtensionType", &InternalExtensionType{})
+	runtime.GetScheme("testExtension").AddKnownTypeWithName("", "A", &ExtensionA{})
+	runtime.GetScheme("testExtension").AddKnownTypeWithName("", "B", &ExtensionB{})
+	runtime.DefaultScheme.AddKnownTypeWithName("testExternal", "ExtensionType", &ExternalExtensionType{})
+	runtime.GetScheme("testExtension").AddKnownTypeWithName("testExternal", "A", &ExtensionA{})
+	runtime.GetScheme("testExtension").AddKnownTypeWithName("testExternal", "B", &ExtensionB{})
+
+	table := []struct {
+		obj     runtime.Object
+		encoded string
+	}{
+		{
+			&InternalExtensionType{Extension: runtime.EmbeddedObject{&ExtensionA{TestString: "foo"}}},
+			`{"kind":"ExtensionType","apiVersion":"testExternal","extension":{"kind":"A","testString":"foo"}}`,
+		}, {
+			&InternalExtensionType{Extension: runtime.EmbeddedObject{&ExtensionB{TestString: "bar"}}},
+			`{"kind":"ExtensionType","apiVersion":"testExternal","extension":{"kind":"B","testString":"bar"}}`,
+		}, {
+			&InternalExtensionType{Extension: runtime.EmbeddedObject{nil}},
+			`{"kind":"ExtensionType","apiVersion":"testExternal","extension":null}`,
+		},
+	}
+
+	for _, item := range table {
+		gotEncoded, err := runtime.DefaultScheme.EncodeToVersion(item.obj, "testExternal")
+		if err != nil {
+			t.Errorf("unexpected error '%v' (%#v)", err, item.obj)
+		} else if e, a := item.encoded, string(gotEncoded); e != a {
+			t.Errorf("expected %v, got %v", e, a)
+		}
+
+		gotDecoded, err := runtime.DefaultScheme.Decode([]byte(item.encoded))
+		if err != nil {
+			t.Errorf("unexpected error '%v' (%v)", err, item.encoded)
+		} else if e, a := item.obj, gotDecoded; !reflect.DeepEqual(e, a) {
+			var eEx, aEx runtime.Object
+			if obj, ok := e.(*InternalExtensionType); ok {
+				eEx = obj.Extension.Object
+			}
+			if obj, ok := a.(*InternalExtensionType); ok {
+				aEx = obj.Extension.Object
+			}
+			t.Errorf("expected %#v, got %#v (%#v, %#v)", e, a, eEx, aEx)
+		}
+	}
+}
+
+func TestExtensionMappingWithoutScheme(t *testing.T) {
+	runtime.DefaultScheme.AddKnownTypeWithName("", "ExtensionType", &InternalExtensionTypeNoScheme{})
+	runtime.DefaultScheme.AddKnownTypeWithName("", "ExA", &ExtensionA{})
+	runtime.DefaultScheme.AddKnownTypeWithName("testExternal", "ExtensionType", &ExternalExtensionType{})
+	runtime.DefaultScheme.AddKnownTypeWithName("testExternal", "ExA", &ExtensionA{})
+
+	table := []struct {
+		obj     runtime.Object
+		encoded string
+	}{
+		{
+			&InternalExtensionTypeNoScheme{Extension: runtime.EmbeddedObject{&ExtensionA{TestString: "foo"}}},
+			`{"kind":"ExtensionType","apiVersion":"testExternal","extension":{"kind":"ExA","testString":"foo"}}`,
+		},
+	}
+
+	for _, item := range table {
+		gotEncoded, err := runtime.DefaultScheme.EncodeToVersion(item.obj, "testExternal")
+		if err != nil {
+			t.Errorf("unexpected error '%v' (%#v)", err, item.obj)
+		} else if e, a := item.encoded, string(gotEncoded); e != a {
+			t.Errorf("expected %v, got %v", e, a)
+		}
+
+		gotDecoded, err := runtime.DefaultScheme.Decode([]byte(item.encoded))
+		if err != nil {
+			t.Errorf("unexpected error '%v' (%v)", err, item.encoded)
+		} else if e, a := item.obj, gotDecoded; !reflect.DeepEqual(e, a) {
+			var eEx, aEx runtime.Object
+			if obj, ok := e.(*InternalExtensionTypeNoScheme); ok {
+				eEx = obj.Extension.Object
+			}
+			if obj, ok := a.(*InternalExtensionTypeNoScheme); ok {
+				aEx = obj.Extension.Object
+			}
+			t.Errorf("expected %#v, got %#v (%#v, %#v)", e, a, eEx, aEx)
+		}
+	}
+}
diff --git a/pkg/runtime/types.go b/pkg/runtime/types.go
index 17148733950..c361906d2cb 100644
--- a/pkg/runtime/types.go
+++ b/pkg/runtime/types.go
@@ -43,6 +43,12 @@ type JSONBase struct {
 	APIVersion        string    `json:"apiVersion,omitempty" yaml:"apiVersion,omitempty"`
 }
 
+// PluginBase is like JSONBase, but it's intended for plugin objects that won't ever be encoded
+// except while embedded in other objects.
+type PluginBase struct {
+	Kind string `json:"kind,omitempty" yaml:"kind,omitempty"`
+}
+
 // EmbeddedObject has appropriate encoder and decoder functions, such that on the wire, it's
 // stored as a []byte, but in memory, the contained object is accessable as an Object
 // via the Get() function. Only valid API objects may be stored via EmbeddedObject.
@@ -51,18 +57,70 @@ type JSONBase struct {
 //
 // Note that object assumes that you've registered all of your api types with the api package.
 //
-// TODO(dbsmith): Stop using runtime.Codec, use the codec appropriate for the conversion (I have a plan).
+// EmbeddedObject and RawExtension can be used together to allow for API object extensions:
+// see the comment for RawExtension.
 type EmbeddedObject struct {
 	Object
 }
 
-// Extension allows api objects with unknown types to be passed-through. This can be used
-// to deal with the API objects from a plug-in. Extension objects still have functioning
-// JSONBase features-- kind, version, resourceVersion, etc.
-// TODO: Not implemented yet
-type Extension struct {
-	JSONBase `yaml:",inline" json:",inline"`
-	// RawJSON to go here.
+// RawExtension is used with EmbeddedObject to do a two-phase encoding of extension objects.
+//
+// To use this, make a field which has RawExtension as its type in your external, versioned
+// struct, and EmbeddedObject in your internal struct. You also need to register your
+// various plugin types.
+//
+// // Internal package:
+// type MyAPIObject struct {
+// 	runtime.JSONBase `yaml:",inline" json:",inline"`
+//	// The "scheme" tag is optional; if absent, runtime.DefaultScheme will be used.
+//	MyPlugin runtime.EmbeddedObject `scheme:"pluginScheme"`
+// }
+// type PluginA struct {
+// 	runtime.PluginBase `yaml:",inline" json:",inline"`
+//	AOption string `yaml:"aOption" json:"aOption"`
+// }
+//
+// // External package:
+// type MyAPIObject struct {
+// 	runtime.JSONBase `yaml:",inline" json:",inline"`
+//	MyPlugin runtime.RawExtension `json:"myPlugin" yaml:"myPlugin"`
+// }
+// type PluginA struct {
+// 	runtime.PluginBase `yaml:",inline" json:",inline"`
+//	AOption string `yaml:"aOption" json:"aOption"`
+// }
+//
+// // On the wire, the JSON will look something like this:
+// {
+//	"kind":"MyAPIObject",
+//	"apiVersion":"v1beta1",
+//	"myPlugin": {
+//		"kind":"PluginA",
+//		"aOption":"foo",
+//	},
+// }
+//
+// So what happens? Decode first uses json or yaml to unmarshal the serialized data into
+// your external MyAPIObject. That causes the raw JSON to be stored, but not unpacked.
+// The next step is to copy (using pkg/conversion) into the internal struct. The runtime
+// package's DefaultScheme has conversion functions installed which will unpack the
+// JSON stored in RawExtension, turning it into the correct object type, and storing it
+// in the EmbeddedObject. (TODO: In the case where the object is of an unknown type, a
+// runtime.Unknown object will be created and stored.)
+type RawExtension struct {
+	RawJSON []byte
 }
 
-func (*Extension) IsAnAPIObject() {}
+// Unknown allows api objects with unknown types to be passed-through. This can be used
+// to deal with the API objects from a plug-in. Unknown objects still have functioning
+// JSONBase features-- kind, version, resourceVersion, etc.
+// TODO: Not implemented yet!
+type Unknown struct {
+	JSONBase `yaml:",inline" json:",inline"`
+	// RawJSON will hold the complete JSON of the object which couldn't be matched
+	// with a registered type. Most likely, nothing should be done with this
+	// except for passing it through the system.
+	RawJSON []byte
+}
+
+func (*Unknown) IsAnAPIObject() {}