diff --git a/testing/prototest/prototest.go b/testing/prototest/prototest.go
index 7d8c91d..7cd1747 100644
--- a/testing/prototest/prototest.go
+++ b/testing/prototest/prototest.go
@@ -319,16 +319,27 @@
 
 // testOneof tests the behavior of fields in a oneof.
 func testOneof(t testing.TB, m pref.Message, od pref.OneofDescriptor) {
-	for i := 0; i < od.Fields().Len(); i++ {
-		fda := od.Fields().Get(i)
-		m.Set(fda, newValue(m, fda, 1, nil))
-		if got, want := m.WhichOneof(od), fda; got != want {
-			t.Errorf("after setting oneof field %q:\nWhichOneof(%q) = %v, want %v", fda.FullName(), fda.Name(), got, want)
-		}
-		for j := 0; j < od.Fields().Len(); j++ {
-			fdb := od.Fields().Get(j)
-			if got, want := m.Has(fdb), i == j; got != want {
-				t.Errorf("after setting oneof field %q:\nGet(%q) = %v, want %v", fda.FullName(), fdb.FullName(), got, want)
+	for _, mutable := range []bool{false, true} {
+		for i := 0; i < od.Fields().Len(); i++ {
+			fda := od.Fields().Get(i)
+			if mutable {
+				// Set fields by requesting a mutable reference.
+				if !fda.IsMap() && !fda.IsList() && fda.Message() == nil {
+					continue
+				}
+				_ = m.Mutable(fda)
+			} else {
+				// Set fields explicitly.
+				m.Set(fda, newValue(m, fda, 1, nil))
+			}
+			if got, want := m.WhichOneof(od), fda; got != want {
+				t.Errorf("after setting oneof field %q:\nWhichOneof(%q) = %v, want %v", fda.FullName(), fda.Name(), got, want)
+			}
+			for j := 0; j < od.Fields().Len(); j++ {
+				fdb := od.Fields().Get(j)
+				if got, want := m.Has(fdb), i == j; got != want {
+					t.Errorf("after setting oneof field %q:\nGet(%q) = %v, want %v", fda.FullName(), fdb.FullName(), got, want)
+				}
 			}
 		}
 	}
diff --git a/types/dynamicpb/dynamic.go b/types/dynamicpb/dynamic.go
new file mode 100644
index 0000000..9d2d6fb
--- /dev/null
+++ b/types/dynamicpb/dynamic.go
@@ -0,0 +1,421 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package dynamicpb creates protocol buffer messages using runtime type information.
+package dynamicpb
+
+import (
+	"math"
+
+	"google.golang.org/protobuf/internal/errors"
+	pref "google.golang.org/protobuf/reflect/protoreflect"
+	protoimpl "google.golang.org/protobuf/runtime/protoimpl"
+)
+
+// A Message is a dynamically constructed protocol buffer message.
+//
+// Message implements the proto.Message interface, and may be used with all
+// standard proto package functions such as Marshal, Unmarshal, and so forth.
+//
+// Message also implements the protoreflect.Message interface. See the protoreflect
+// package documentation for that interface for how to get and set fields and
+// otherwise interact with the contents of a Message.
+//
+// Reflection API functions which construct messages, such as NewMessage,
+// return new dynamic messages of the appropriate type. Functions which take
+// messages, such as Set for a message-value field, will accept any message
+// with a compatible type.
+//
+// Operations which modify a Message are not safe for concurrent use.
+type Message struct {
+	desc    pref.MessageDescriptor
+	known   map[pref.FieldNumber]pref.Value
+	ext     map[pref.FieldNumber]pref.FieldDescriptor
+	unknown pref.RawFields
+}
+
+// New creates a new message with the provided descriptor.
+func New(desc pref.MessageDescriptor) *Message {
+	return &Message{
+		desc:  desc,
+		known: make(map[pref.FieldNumber]pref.Value),
+		ext:   make(map[pref.FieldNumber]pref.FieldDescriptor),
+	}
+}
+
+// ProtoReflect implements the protoreflect.ProtoMessage interface.
+func (m *Message) ProtoReflect() pref.Message {
+	return m
+}
+
+// String returns a string representation of a message.
+func (m *Message) String() string {
+	return protoimpl.X.MessageStringOf(m)
+}
+
+// Descriptor returns the message descriptor.
+func (m *Message) Descriptor() pref.MessageDescriptor {
+	return m.desc
+}
+
+// New returns a newly allocated empty message with the same descriptor.
+// See protoreflect.Message for details.
+func (m *Message) New() pref.Message {
+	return New(m.desc)
+}
+
+// Interface returns the message.
+// See protoreflect.Message for details.
+func (m *Message) Interface() pref.ProtoMessage {
+	return m
+}
+
+// Len returns the number of populated fields.
+// See protoreflect.Message for details.
+func (m *Message) Len() int {
+	count := 0
+	for num, v := range m.known {
+		if m.ext[num] != nil {
+			count++
+			continue
+		}
+		if isSet(m.desc.Fields().ByNumber(num), v) {
+			count++
+		}
+	}
+	return count
+}
+
+// Range visits every populated field in undefined order.
+// See protoreflect.Message for details.
+func (m *Message) Range(f func(pref.FieldDescriptor, pref.Value) bool) {
+	for num, v := range m.known {
+		fd := m.ext[num]
+		if fd == nil {
+			fd = m.desc.Fields().ByNumber(num)
+			if !isSet(fd, v) {
+				continue
+			}
+		}
+		if !f(fd, v) {
+			return
+		}
+	}
+}
+
+// Range reports whether a field is populated.
+// See protoreflect.Message for details.
+func (m *Message) Has(fd pref.FieldDescriptor) bool {
+	m.checkField(fd)
+	if fd.IsExtension() {
+		return m.ext[fd.Number()] == fd
+	}
+	v, ok := m.known[fd.Number()]
+	if !ok {
+		return false
+	}
+	return isSet(fd, v)
+}
+
+// Clear clears a field.
+// See protoreflect.Message for details.
+func (m *Message) Clear(fd pref.FieldDescriptor) {
+	m.checkField(fd)
+	num := fd.Number()
+	delete(m.known, num)
+	delete(m.ext, num)
+}
+
+// Get returns the value of a field.
+// See protoreflect.Message for details.
+func (m *Message) Get(fd pref.FieldDescriptor) pref.Value {
+	m.checkField(fd)
+	num := fd.Number()
+	if v, ok := m.known[num]; ok {
+		if !fd.IsExtension() || fd == m.ext[num] {
+			return v
+		}
+	}
+	switch {
+	case fd.IsMap():
+		return pref.ValueOf(&dynamicMap{desc: fd})
+	case fd.Cardinality() == pref.Repeated:
+		return pref.ValueOf(emptyList{desc: fd})
+	case fd.Kind() == pref.BytesKind:
+		return pref.ValueOf(append([]byte(nil), fd.Default().Bytes()...))
+	default:
+		return fd.Default()
+	}
+}
+
+// Mutable returns a mutable reference to a repeated, map, or message field.
+// See protoreflect.Message for details.
+func (m *Message) Mutable(fd pref.FieldDescriptor) pref.Value {
+	m.checkField(fd)
+	num := fd.Number()
+	if v, ok := m.known[num]; ok {
+		if !fd.IsExtension() || fd == m.ext[num] {
+			return v
+		}
+	}
+	if !fd.IsMap() && !fd.IsList() && fd.Message() == nil {
+		panic(errors.New("%v: getting mutable reference to non-composite type", fd.FullName()))
+	}
+	m.clearOtherOneofFields(fd)
+	switch {
+	case fd.IsExtension():
+		m.known[num] = fd.(pref.ExtensionType).New()
+		m.ext[num] = fd
+	case fd.IsMap():
+		m.known[num] = pref.ValueOf(&dynamicMap{
+			desc: fd,
+			mapv: make(map[interface{}]pref.Value),
+		})
+	case fd.IsList():
+		m.known[num] = pref.ValueOf(&dynamicList{desc: fd})
+	case fd.Message() != nil:
+		m.known[num] = pref.ValueOf(m.NewMessage(fd))
+	}
+	return m.known[num]
+}
+
+// Set stores a value in a field.
+// See protoreflect.Message for details.
+func (m *Message) Set(fd pref.FieldDescriptor, v pref.Value) {
+	m.checkField(fd)
+	switch {
+	case fd.IsExtension():
+		// Call InterfaceOf just to let the extension typecheck the value.
+		_ = fd.(pref.ExtensionType).InterfaceOf(v)
+		m.ext[fd.Number()] = fd
+	case fd.IsMap():
+		if mapv, ok := v.Interface().(*dynamicMap); !ok || mapv.desc != fd {
+			panic(errors.New("%v: assigning invalid type %T", fd.FullName(), v.Interface()))
+		}
+	case fd.IsList():
+		if list, ok := v.Interface().(*dynamicList); !ok || list.desc != fd {
+			panic(errors.New("%v: assigning invalid type %T", fd.FullName(), v.Interface()))
+		}
+	default:
+		typecheckSingular(fd, v)
+	}
+	m.clearOtherOneofFields(fd)
+	m.known[fd.Number()] = v
+}
+
+func (m *Message) clearOtherOneofFields(fd pref.FieldDescriptor) {
+	od := fd.ContainingOneof()
+	if od == nil {
+		return
+	}
+	num := fd.Number()
+	for i := 0; i < od.Fields().Len(); i++ {
+		if n := od.Fields().Get(i).Number(); n != num {
+			delete(m.known, n)
+		}
+	}
+}
+
+// NewMessage returns a newly-allocated message assignable to a field.
+// See protoreflect.Message for details.
+func (m *Message) NewMessage(fd pref.FieldDescriptor) pref.Message {
+	m.checkField(fd)
+	md := fd.Message()
+	if fd.Cardinality() == pref.Repeated || md == nil {
+		panic(errors.New("%v: field is not of non-repeated message type", fd.FullName()))
+	}
+	return New(md).ProtoReflect()
+}
+
+// WhichOneof reports which field in a oneof is populated, returning nil if none are populated.
+// See protoreflect.Message for details.
+func (m *Message) WhichOneof(od pref.OneofDescriptor) pref.FieldDescriptor {
+	for i := 0; i < od.Fields().Len(); i++ {
+		fd := od.Fields().Get(i)
+		if m.Has(fd) {
+			return fd
+		}
+	}
+	return nil
+}
+
+// GetUnknown returns the raw unknown fields.
+// See protoreflect.Message for details.
+func (m *Message) GetUnknown() pref.RawFields {
+	return m.unknown
+}
+
+// SetUnknown sets the raw unknown fields.
+// See protoreflect.Message for details.
+func (m *Message) SetUnknown(r pref.RawFields) {
+	m.unknown = r
+}
+
+func (m *Message) checkField(fd pref.FieldDescriptor) {
+	if fd.IsExtension() && fd.ContainingMessage().FullName() == m.desc.FullName() {
+		if _, ok := fd.(pref.ExtensionType); !ok {
+			panic(errors.New("%v: extension field descriptor does not implement ExtensionType", fd.FullName()))
+		}
+		return
+	}
+	if fd.Parent() == m.desc {
+		return
+	}
+	fields := m.desc.Fields()
+	index := fd.Index()
+	if index >= fields.Len() || fields.Get(index) != fd {
+		panic(errors.New("%v: field descriptor does not belong to this message", fd.FullName()))
+	}
+}
+
+type emptyList struct {
+	desc pref.FieldDescriptor
+}
+
+func (x emptyList) Len() int                { return 0 }
+func (x emptyList) Get(n int) pref.Value    { panic(errors.New("out of range")) }
+func (x emptyList) Set(n int, v pref.Value) { panic(errors.New("modification of immutable list")) }
+func (x emptyList) Append(v pref.Value)     { panic(errors.New("modification of immutable list")) }
+func (x emptyList) Truncate(n int)          { panic(errors.New("modification of immutable list")) }
+func (x emptyList) NewMessage() pref.Message {
+	md := x.desc.Message()
+	if md == nil {
+		panic(errors.New("list is not of message type"))
+	}
+	return New(md).ProtoReflect()
+}
+
+type dynamicList struct {
+	desc pref.FieldDescriptor
+	list []pref.Value
+}
+
+func (x *dynamicList) Len() int {
+	return len(x.list)
+}
+
+func (x *dynamicList) Get(n int) pref.Value {
+	return x.list[n]
+}
+
+func (x *dynamicList) Set(n int, v pref.Value) {
+	typecheckSingular(x.desc, v)
+	x.list[n] = v
+}
+
+func (x *dynamicList) Append(v pref.Value) {
+	typecheckSingular(x.desc, v)
+	x.list = append(x.list, v)
+}
+
+func (x *dynamicList) Truncate(n int) {
+	// Zero truncated elements to avoid keeping data live.
+	for i := n; i < len(x.list); i++ {
+		x.list[i] = pref.Value{}
+	}
+	x.list = x.list[:n]
+}
+
+func (x *dynamicList) NewMessage() pref.Message {
+	md := x.desc.Message()
+	if md == nil {
+		panic(errors.New("list is not of message type"))
+	}
+	return New(md).ProtoReflect()
+}
+
+type dynamicMap struct {
+	desc pref.FieldDescriptor
+	mapv map[interface{}]pref.Value
+}
+
+func (x *dynamicMap) Get(k pref.MapKey) pref.Value { return x.mapv[k.Interface()] }
+func (x *dynamicMap) Set(k pref.MapKey, v pref.Value) {
+	typecheckSingular(x.desc.MapKey(), k.Value())
+	typecheckSingular(x.desc.MapValue(), v)
+	x.mapv[k.Interface()] = v
+}
+func (x *dynamicMap) Has(k pref.MapKey) bool { return x.Get(k).IsValid() }
+func (x *dynamicMap) Clear(k pref.MapKey)    { delete(x.mapv, k.Interface()) }
+func (x *dynamicMap) Len() int               { return len(x.mapv) }
+func (x *dynamicMap) NewMessage() pref.Message {
+	md := x.desc.MapValue().Message()
+	if md == nil {
+		panic(errors.New("map value is not of message type"))
+	}
+	return New(md).ProtoReflect()
+}
+func (x *dynamicMap) Range(f func(pref.MapKey, pref.Value) bool) {
+	for k, v := range x.mapv {
+		if !f(pref.ValueOf(k).MapKey(), v) {
+			return
+		}
+	}
+}
+
+func isSet(fd pref.FieldDescriptor, v pref.Value) bool {
+	switch {
+	case fd.IsMap():
+		return v.Map().Len() > 0
+	case fd.IsList():
+		return v.List().Len() > 0
+	case fd.ContainingOneof() != nil:
+		return true
+	case fd.Syntax() == pref.Proto3:
+		switch fd.Kind() {
+		case pref.BoolKind:
+			return v.Bool()
+		case pref.EnumKind:
+			return v.Enum() != 0
+		case pref.Int32Kind, pref.Sint32Kind, pref.Int64Kind, pref.Sint64Kind, pref.Sfixed32Kind, pref.Sfixed64Kind:
+			return v.Int() != 0
+		case pref.Uint32Kind, pref.Uint64Kind, pref.Fixed32Kind, pref.Fixed64Kind:
+			return v.Uint() != 0
+		case pref.FloatKind, pref.DoubleKind:
+			return v.Float() != 0 || math.Signbit(v.Float())
+		case pref.StringKind:
+			return v.String() != ""
+		case pref.BytesKind:
+			return len(v.Bytes()) > 0
+		}
+	}
+	return true
+}
+
+func typecheckSingular(fd pref.FieldDescriptor, v pref.Value) {
+	vi := v.Interface()
+	var ok bool
+	switch fd.Kind() {
+	case pref.BoolKind:
+		_, ok = vi.(bool)
+	case pref.EnumKind:
+		// We could check against the valid set of enum values, but do not.
+		_, ok = vi.(pref.EnumNumber)
+	case pref.Int32Kind, pref.Sint32Kind, pref.Sfixed32Kind:
+		_, ok = vi.(int32)
+	case pref.Uint32Kind, pref.Fixed32Kind:
+		_, ok = vi.(uint32)
+	case pref.Int64Kind, pref.Sint64Kind, pref.Sfixed64Kind:
+		_, ok = vi.(int64)
+	case pref.Uint64Kind, pref.Fixed64Kind:
+		_, ok = vi.(uint64)
+	case pref.FloatKind:
+		_, ok = vi.(float32)
+	case pref.DoubleKind:
+		_, ok = vi.(float64)
+	case pref.StringKind:
+		_, ok = vi.(string)
+	case pref.BytesKind:
+		_, ok = vi.([]byte)
+	case pref.MessageKind, pref.GroupKind:
+		var m pref.Message
+		m, ok = vi.(pref.Message)
+		if ok && m.Descriptor().FullName() != fd.Message().FullName() {
+			panic(errors.New("%v: assigning invalid message type %v", fd.FullName(), m.Descriptor().FullName()))
+		}
+	}
+	if !ok {
+		panic(errors.New("%v: assigning invalid type %T", fd.FullName(), v.Interface()))
+	}
+}
diff --git a/types/dynamicpb/dynamic_test.go b/types/dynamicpb/dynamic_test.go
new file mode 100644
index 0000000..5a7db95
--- /dev/null
+++ b/types/dynamicpb/dynamic_test.go
@@ -0,0 +1,26 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dynamicpb_test
+
+import (
+	"testing"
+
+	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/testing/prototest"
+	"google.golang.org/protobuf/types/dynamicpb"
+
+	testpb "google.golang.org/protobuf/internal/testprotos/test"
+	test3pb "google.golang.org/protobuf/internal/testprotos/test3"
+)
+
+func TestConformance(t *testing.T) {
+	for _, message := range []proto.Message{
+		(*testpb.TestAllTypes)(nil),
+		(*test3pb.TestAllTypes)(nil),
+		(*testpb.TestAllExtensions)(nil),
+	} {
+		prototest.TestMessage(t, dynamicpb.New(message.ProtoReflect().Descriptor()), prototest.MessageOptions{})
+	}
+}