diff --git a/internal/cmd/generate-types/main.go b/internal/cmd/generate-types/main.go
index 0c0a34d..dfca236 100644
--- a/internal/cmd/generate-types/main.go
+++ b/internal/cmd/generate-types/main.go
@@ -39,8 +39,9 @@
 
 	chdirRoot()
 	writeSource("internal/fileinit/desc_list_gen.go", generateFileinitDescList())
-	writeSource("proto/decode_gen.go", generateProtoDecode())
 	writeSource("internal/prototype/protofile_list_gen.go", generateListTypes())
+	writeSource("proto/decode_gen.go", generateProtoDecode())
+	writeSource("proto/encode_gen.go", generateProtoEncode())
 }
 
 // chdirRoot changes the working directory to the repository root.
@@ -304,6 +305,7 @@
 		"sync",
 		"",
 		"github.com/golang/protobuf/v2/internal/encoding/wire",
+		"github.com/golang/protobuf/v2/internal/errors",
 		"github.com/golang/protobuf/v2/internal/pragma",
 		"github.com/golang/protobuf/v2/internal/typefmt",
 		"github.com/golang/protobuf/v2/reflect/protoreflect",
diff --git a/internal/cmd/generate-types/proto.go b/internal/cmd/generate-types/proto.go
index c35e621..dea9581 100644
--- a/internal/cmd/generate-types/proto.go
+++ b/internal/cmd/generate-types/proto.go
@@ -28,9 +28,10 @@
 }
 
 type ProtoKind struct {
-	Name     string
-	WireType WireType
-	ToValue  Expr
+	Name      string
+	WireType  WireType
+	ToValue   Expr
+	FromValue Expr
 }
 
 func (k ProtoKind) Expr() Expr {
@@ -39,94 +40,112 @@
 
 var ProtoKinds = []ProtoKind{
 	{
-		Name:     "Bool",
-		WireType: WireVarint,
-		ToValue:  "wire.DecodeBool(v)",
+		Name:      "Bool",
+		WireType:  WireVarint,
+		ToValue:   "wire.DecodeBool(v)",
+		FromValue: "wire.EncodeBool(v.Bool())",
 	},
 	{
-		Name:     "Enum",
-		WireType: WireVarint,
-		ToValue:  "protoreflect.EnumNumber(v)",
+		Name:      "Enum",
+		WireType:  WireVarint,
+		ToValue:   "protoreflect.EnumNumber(v)",
+		FromValue: "uint64(v.Enum())",
 	},
 	{
-		Name:     "Int32",
-		WireType: WireVarint,
-		ToValue:  "int32(v)",
+		Name:      "Int32",
+		WireType:  WireVarint,
+		ToValue:   "int32(v)",
+		FromValue: "uint64(int32(v.Int()))",
 	},
 	{
-		Name:     "Sint32",
-		WireType: WireVarint,
-		ToValue:  "int32(wire.DecodeZigZag(v & math.MaxUint32))",
+		Name:      "Sint32",
+		WireType:  WireVarint,
+		ToValue:   "int32(wire.DecodeZigZag(v & math.MaxUint32))",
+		FromValue: "wire.EncodeZigZag(int64(int32(v.Int())))",
 	},
 	{
-		Name:     "Uint32",
-		WireType: WireVarint,
-		ToValue:  "uint32(v)",
+		Name:      "Uint32",
+		WireType:  WireVarint,
+		ToValue:   "uint32(v)",
+		FromValue: "uint64(uint32(v.Uint()))",
 	},
 	{
-		Name:     "Int64",
-		WireType: WireVarint,
-		ToValue:  "int64(v)",
+		Name:      "Int64",
+		WireType:  WireVarint,
+		ToValue:   "int64(v)",
+		FromValue: "uint64(v.Int())",
 	},
 	{
-		Name:     "Sint64",
-		WireType: WireVarint,
-		ToValue:  "wire.DecodeZigZag(v)",
+		Name:      "Sint64",
+		WireType:  WireVarint,
+		ToValue:   "wire.DecodeZigZag(v)",
+		FromValue: "wire.EncodeZigZag(v.Int())",
 	},
 	{
-		Name:     "Uint64",
-		WireType: WireVarint,
-		ToValue:  "v",
+		Name:      "Uint64",
+		WireType:  WireVarint,
+		ToValue:   "v",
+		FromValue: "v.Uint()",
 	},
 	{
-		Name:     "Sfixed32",
-		WireType: WireFixed32,
-		ToValue:  "int32(v)",
+		Name:      "Sfixed32",
+		WireType:  WireFixed32,
+		ToValue:   "int32(v)",
+		FromValue: "uint32(v.Int())",
 	},
 	{
-		Name:     "Fixed32",
-		WireType: WireFixed32,
-		ToValue:  "uint32(v)",
+		Name:      "Fixed32",
+		WireType:  WireFixed32,
+		ToValue:   "uint32(v)",
+		FromValue: "uint32(v.Uint())",
 	},
 	{
-		Name:     "Float",
-		WireType: WireFixed32,
-		ToValue:  "math.Float32frombits(uint32(v))",
+		Name:      "Float",
+		WireType:  WireFixed32,
+		ToValue:   "math.Float32frombits(uint32(v))",
+		FromValue: "math.Float32bits(float32(v.Float()))",
 	},
 	{
-		Name:     "Sfixed64",
-		WireType: WireFixed64,
-		ToValue:  "int64(v)",
+		Name:      "Sfixed64",
+		WireType:  WireFixed64,
+		ToValue:   "int64(v)",
+		FromValue: "uint64(v.Int())",
 	},
 	{
-		Name:     "Fixed64",
-		WireType: WireFixed64,
-		ToValue:  "v",
+		Name:      "Fixed64",
+		WireType:  WireFixed64,
+		ToValue:   "v",
+		FromValue: "v.Uint()",
 	},
 	{
-		Name:     "Double",
-		WireType: WireFixed64,
-		ToValue:  "math.Float64frombits(v)",
+		Name:      "Double",
+		WireType:  WireFixed64,
+		ToValue:   "math.Float64frombits(v)",
+		FromValue: "math.Float64bits(v.Float())",
 	},
 	{
-		Name:     "String",
-		WireType: WireBytes,
-		ToValue:  "string(v)",
+		Name:      "String",
+		WireType:  WireBytes,
+		ToValue:   "string(v)",
+		FromValue: "[]byte(v.String())",
 	},
 	{
-		Name:     "Bytes",
-		WireType: WireBytes,
-		ToValue:  "append(([]byte)(nil), v...)",
+		Name:      "Bytes",
+		WireType:  WireBytes,
+		ToValue:   "append(([]byte)(nil), v...)",
+		FromValue: "v.Bytes()",
 	},
 	{
-		Name:     "Message",
-		WireType: WireBytes,
-		ToValue:  "v",
+		Name:      "Message",
+		WireType:  WireBytes,
+		ToValue:   "v",
+		FromValue: "v",
 	},
 	{
-		Name:     "Group",
-		WireType: WireGroup,
-		ToValue:  "v",
+		Name:      "Group",
+		WireType:  WireGroup,
+		ToValue:   "v",
+		FromValue: "v",
 	},
 }
 
@@ -208,3 +227,45 @@
 	}
 }
 `))
+
+func generateProtoEncode() string {
+	return mustExecute(protoEncodeTemplate, ProtoKinds)
+}
+
+var protoEncodeTemplate = template.Must(template.New("").Parse(`
+var wireTypes = map[protoreflect.Kind]wire.Type{
+{{- range .}}
+	{{.Expr}}: {{.WireType.Expr}},
+{{- end}}
+}
+
+func (o MarshalOptions) marshalSingular(b []byte, num wire.Number, kind protoreflect.Kind, v protoreflect.Value) ([]byte, error) {
+	switch kind {
+	{{- range .}}
+	case {{.Expr}}:
+		{{if (eq .Name "Message") -}}
+		var pos int
+		var err error
+		b, pos = appendSpeculativeLength(b)
+		b, err = o.marshalMessage(b, v.Message())
+		if err != nil {
+			return nil, err
+		}
+		b = finishSpeculativeLength(b, pos)
+		{{- else if (eq .Name "Group") -}}
+		var err error
+		b, err = o.marshalMessage(b, v.Message())
+		if err != nil {
+			return nil, err
+		}
+		b = wire.AppendVarint(b, wire.EncodeTag(num, wire.EndGroupType))
+		{{- else -}}
+		b = wire.Append{{.WireType}}(b, {{.FromValue}})
+		{{- end}}
+	{{- end}}
+	default:
+		return nil, errors.New("invalid kind %v", kind)
+	}
+	return b, nil
+}
+`))
diff --git a/internal/mapsort/mapsort.go b/internal/mapsort/mapsort.go
new file mode 100644
index 0000000..8999b50
--- /dev/null
+++ b/internal/mapsort/mapsort.go
@@ -0,0 +1,47 @@
+// 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 mapsort provides sorted access to maps.
+package mapsort
+
+import (
+	"sort"
+
+	"github.com/golang/protobuf/v2/reflect/protoreflect"
+)
+
+// Range iterates over every map entry in sorted key order,
+// calling f for each key and value encountered.
+func Range(mapv protoreflect.Map, keyKind protoreflect.Kind, f func(protoreflect.MapKey, protoreflect.Value) bool) {
+	var keys []protoreflect.MapKey
+	mapv.Range(func(key protoreflect.MapKey, _ protoreflect.Value) bool {
+		keys = append(keys, key)
+		return true
+	})
+	sort.Slice(keys, func(i, j int) bool {
+		switch keyKind {
+		case protoreflect.BoolKind:
+			return !keys[i].Bool() && keys[j].Bool()
+		case protoreflect.Int32Kind,
+			protoreflect.Sint32Kind,
+			protoreflect.Sfixed32Kind,
+			protoreflect.Int64Kind,
+			protoreflect.Sint64Kind,
+			protoreflect.Sfixed64Kind:
+			return keys[i].Int() < keys[j].Int()
+		case protoreflect.Uint32Kind,
+			protoreflect.Fixed32Kind,
+			protoreflect.Uint64Kind,
+			protoreflect.Fixed64Kind:
+			return keys[i].Uint() < keys[j].Uint()
+		default:
+			return keys[i].String() < keys[j].String()
+		}
+	})
+	for _, key := range keys {
+		if !f(key, mapv.Get(key)) {
+			break
+		}
+	}
+}
diff --git a/internal/mapsort/mapsort_test.go b/internal/mapsort/mapsort_test.go
new file mode 100644
index 0000000..680dcb8
--- /dev/null
+++ b/internal/mapsort/mapsort_test.go
@@ -0,0 +1,67 @@
+// 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 mapsort_test
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/golang/protobuf/v2/internal/mapsort"
+	"github.com/golang/protobuf/v2/internal/value"
+	pref "github.com/golang/protobuf/v2/reflect/protoreflect"
+)
+
+func TestRange(t *testing.T) {
+	for _, test := range []struct {
+		mapv interface{}
+		kind pref.Kind
+	}{
+		{
+			mapv: &map[bool]int32{
+				false: 0,
+				true:  1,
+			},
+			kind: pref.BoolKind,
+		},
+		{
+			mapv: &map[int32]int32{
+				0: 0,
+				1: 1,
+				2: 2,
+			},
+			kind: pref.Int32Kind,
+		},
+		{
+			mapv: &map[uint64]int32{
+				0: 0,
+				1: 1,
+				2: 2,
+			},
+			kind: pref.Uint64Kind,
+		},
+		{
+			mapv: &map[string]int32{
+				"a": 0,
+				"b": 1,
+				"c": 2,
+			},
+			kind: pref.StringKind,
+		},
+	} {
+		rv := reflect.TypeOf(test.mapv).Elem()
+		mapv := value.MapOf(test.mapv, value.NewConverter(rv.Key(), test.kind), value.NewConverter(rv.Elem(), pref.Int32Kind))
+		var got []pref.MapKey
+		mapsort.Range(mapv, test.kind, func(key pref.MapKey, _ pref.Value) bool {
+			got = append(got, key)
+			return true
+		})
+		for i, key := range got {
+			if int64(i) != mapv.Get(key).Int() {
+				t.Errorf("out of order range over map: %v", got)
+				break
+			}
+		}
+	}
+}
diff --git a/proto/encode.go b/proto/encode.go
new file mode 100644
index 0000000..68a3448
--- /dev/null
+++ b/proto/encode.go
@@ -0,0 +1,216 @@
+// 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 proto
+
+import (
+	"fmt"
+	"sort"
+
+	"github.com/golang/protobuf/v2/internal/encoding/wire"
+	"github.com/golang/protobuf/v2/internal/mapsort"
+	"github.com/golang/protobuf/v2/internal/pragma"
+	"github.com/golang/protobuf/v2/reflect/protoreflect"
+)
+
+// MarshalOptions configures the marshaler.
+//
+// Example usage:
+//   b, err := MarshalOptions{Deterministic: true}.Marshal(m)
+type MarshalOptions struct {
+	// Deterministic controls whether the same message will always be
+	// serialized to the same bytes within the same binary.
+	//
+	// Setting this option guarantees that repeated serialization of
+	// the same message will return the same bytes, and that different
+	// processes of the same binary (which may be executing on different
+	// machines) will serialize equal messages to the same bytes.
+	//
+	// Note that the deterministic serialization is NOT canonical across
+	// languages. It is not guaranteed to remain stable over time. It is
+	// unstable across different builds with schema changes due to unknown
+	// fields. Users who need canonical serialization (e.g., persistent
+	// storage in a canonical form, fingerprinting, etc.) must define
+	// their own canonicalization specification and implement their own
+	// serializer rather than relying on this API.
+	//
+	// If deterministic serialization is requested, map entries will be
+	// sorted by keys in lexographical order. This is an implementation
+	// detail and subject to change.
+	Deterministic bool
+
+	pragma.NoUnkeyedLiterals
+}
+
+// Marshal returns the wire-format encoding of m.
+func Marshal(m Message) ([]byte, error) {
+	return MarshalOptions{}.MarshalAppend(nil, m)
+}
+
+// Marshal returns the wire-format encoding of m.
+func (o MarshalOptions) Marshal(m Message) ([]byte, error) {
+	return o.marshalMessage(nil, m.ProtoReflect())
+}
+
+// MarshalAppend appends the wire-format encoding of m to b,
+// returning the result.
+func (o MarshalOptions) MarshalAppend(b []byte, m Message) ([]byte, error) {
+	return o.marshalMessage(b, m.ProtoReflect())
+}
+
+func (o MarshalOptions) marshalMessage(b []byte, m protoreflect.Message) ([]byte, error) {
+	// There are many choices for what order we visit fields in. The default one here
+	// is chosen for reasonable efficiency and simplicity given the protoreflect API.
+	// It is not deterministic, since KnownFields.Range does not return fields in any
+	// defined order.
+	//
+	// When using deterministic serialization, we sort the known fields by field number.
+	fields := m.Type().Fields()
+	knownFields := m.KnownFields()
+	var err error
+	o.rangeKnown(knownFields, func(num protoreflect.FieldNumber, value protoreflect.Value) bool {
+		field := fields.ByNumber(num)
+		if field == nil {
+			field = knownFields.ExtensionTypes().ByNumber(num)
+			if field == nil {
+				panic(fmt.Errorf("no descriptor for field %d in %q", num, m.Type().FullName()))
+			}
+		}
+		b, err = o.marshalField(b, field, value)
+		return err == nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	m.UnknownFields().Range(func(_ protoreflect.FieldNumber, raw protoreflect.RawFields) bool {
+		b = append(b, raw...)
+		return true
+	})
+	// TODO: required field checks
+	return b, nil
+}
+
+// rangeKnown visits known fields in field number order when deterministic
+// serialization is enabled.
+func (o MarshalOptions) rangeKnown(knownFields protoreflect.KnownFields, f func(protoreflect.FieldNumber, protoreflect.Value) bool) {
+	if !o.Deterministic {
+		knownFields.Range(f)
+		return
+	}
+	nums := make([]protoreflect.FieldNumber, 0, knownFields.Len())
+	knownFields.Range(func(num protoreflect.FieldNumber, _ protoreflect.Value) bool {
+		nums = append(nums, num)
+		return true
+	})
+	sort.Slice(nums, func(a, b int) bool {
+		return nums[a] < nums[b]
+	})
+	for _, num := range nums {
+		if !f(num, knownFields.Get(num)) {
+			break
+		}
+	}
+}
+
+func (o MarshalOptions) marshalField(b []byte, field protoreflect.FieldDescriptor, value protoreflect.Value) ([]byte, error) {
+	num := field.Number()
+	kind := field.Kind()
+	switch {
+	case field.Cardinality() != protoreflect.Repeated:
+		b = wire.AppendTag(b, num, wireTypes[kind])
+		return o.marshalSingular(b, num, kind, value)
+	case field.IsMap():
+		return o.marshalMap(b, num, kind, field.MessageType(), value.Map())
+	case field.IsPacked():
+		return o.marshalPacked(b, num, kind, value.List())
+	default:
+		return o.marshalList(b, num, kind, value.List())
+	}
+}
+
+func (o MarshalOptions) marshalMap(b []byte, num wire.Number, kind protoreflect.Kind, mdesc protoreflect.MessageDescriptor, mapv protoreflect.Map) ([]byte, error) {
+	keyf := mdesc.Fields().ByNumber(1)
+	valf := mdesc.Fields().ByNumber(2)
+	var err error
+	o.rangeMap(mapv, keyf.Kind(), func(key protoreflect.MapKey, value protoreflect.Value) bool {
+		b = wire.AppendTag(b, num, wire.BytesType)
+		var pos int
+		b, pos = appendSpeculativeLength(b)
+
+		b, err = o.marshalField(b, keyf, key.Value())
+		if err != nil {
+			return false
+		}
+		b, err = o.marshalField(b, valf, value)
+		if err != nil {
+			return false
+		}
+
+		b = finishSpeculativeLength(b, pos)
+		return true
+	})
+	if err != nil {
+		return nil, err
+	}
+	return b, nil
+}
+
+func (o MarshalOptions) rangeMap(mapv protoreflect.Map, kind protoreflect.Kind, f func(protoreflect.MapKey, protoreflect.Value) bool) {
+	if !o.Deterministic {
+		mapv.Range(f)
+		return
+	}
+	mapsort.Range(mapv, kind, f)
+}
+
+func (o MarshalOptions) marshalPacked(b []byte, num wire.Number, kind protoreflect.Kind, list protoreflect.List) ([]byte, error) {
+	b = wire.AppendTag(b, num, wire.BytesType)
+	b, pos := appendSpeculativeLength(b)
+	for i, llen := 0, list.Len(); i < llen; i++ {
+		var err error
+		b, err = o.marshalSingular(b, num, kind, list.Get(i))
+		if err != nil {
+			return nil, err
+		}
+	}
+	b = finishSpeculativeLength(b, pos)
+	return b, nil
+}
+
+func (o MarshalOptions) marshalList(b []byte, num wire.Number, kind protoreflect.Kind, list protoreflect.List) ([]byte, error) {
+	for i, llen := 0, list.Len(); i < llen; i++ {
+		var err error
+		b = wire.AppendTag(b, num, wireTypes[kind])
+		b, err = o.marshalSingular(b, num, kind, list.Get(i))
+		if err != nil {
+			return nil, err
+		}
+	}
+	return b, nil
+}
+
+// When encoding length-prefixed fields, we speculatively set aside some number of bytes
+// for the length, encode the data, and then encode the length (shifting the data if necessary
+// to make room).
+const speculativeLength = 1
+
+func appendSpeculativeLength(b []byte) ([]byte, int) {
+	pos := len(b)
+	b = append(b, "\x00\x00\x00\x00"[:speculativeLength]...)
+	return b, pos
+}
+
+func finishSpeculativeLength(b []byte, pos int) []byte {
+	mlen := len(b) - pos - speculativeLength
+	msiz := wire.SizeVarint(uint64(mlen))
+	if msiz != speculativeLength {
+		for i := 0; i < msiz-speculativeLength; i++ {
+			b = append(b, 0)
+		}
+		copy(b[pos+msiz:], b[pos+speculativeLength:])
+		b = b[:pos+msiz+mlen]
+	}
+	wire.AppendVarint(b[:pos], uint64(mlen))
+	return b
+}
diff --git a/proto/encode_gen.go b/proto/encode_gen.go
new file mode 100644
index 0000000..d262133
--- /dev/null
+++ b/proto/encode_gen.go
@@ -0,0 +1,92 @@
+// Copyright 2018 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.
+
+// Code generated by generate-types. DO NOT EDIT.
+
+package proto
+
+import (
+	"math"
+
+	"github.com/golang/protobuf/v2/internal/encoding/wire"
+	"github.com/golang/protobuf/v2/internal/errors"
+	"github.com/golang/protobuf/v2/reflect/protoreflect"
+)
+
+var wireTypes = map[protoreflect.Kind]wire.Type{
+	protoreflect.BoolKind:     wire.VarintType,
+	protoreflect.EnumKind:     wire.VarintType,
+	protoreflect.Int32Kind:    wire.VarintType,
+	protoreflect.Sint32Kind:   wire.VarintType,
+	protoreflect.Uint32Kind:   wire.VarintType,
+	protoreflect.Int64Kind:    wire.VarintType,
+	protoreflect.Sint64Kind:   wire.VarintType,
+	protoreflect.Uint64Kind:   wire.VarintType,
+	protoreflect.Sfixed32Kind: wire.Fixed32Type,
+	protoreflect.Fixed32Kind:  wire.Fixed32Type,
+	protoreflect.FloatKind:    wire.Fixed32Type,
+	protoreflect.Sfixed64Kind: wire.Fixed64Type,
+	protoreflect.Fixed64Kind:  wire.Fixed64Type,
+	protoreflect.DoubleKind:   wire.Fixed64Type,
+	protoreflect.StringKind:   wire.BytesType,
+	protoreflect.BytesKind:    wire.BytesType,
+	protoreflect.MessageKind:  wire.BytesType,
+	protoreflect.GroupKind:    wire.StartGroupType,
+}
+
+func (o MarshalOptions) marshalSingular(b []byte, num wire.Number, kind protoreflect.Kind, v protoreflect.Value) ([]byte, error) {
+	switch kind {
+	case protoreflect.BoolKind:
+		b = wire.AppendVarint(b, wire.EncodeBool(v.Bool()))
+	case protoreflect.EnumKind:
+		b = wire.AppendVarint(b, uint64(v.Enum()))
+	case protoreflect.Int32Kind:
+		b = wire.AppendVarint(b, uint64(int32(v.Int())))
+	case protoreflect.Sint32Kind:
+		b = wire.AppendVarint(b, wire.EncodeZigZag(int64(int32(v.Int()))))
+	case protoreflect.Uint32Kind:
+		b = wire.AppendVarint(b, uint64(uint32(v.Uint())))
+	case protoreflect.Int64Kind:
+		b = wire.AppendVarint(b, uint64(v.Int()))
+	case protoreflect.Sint64Kind:
+		b = wire.AppendVarint(b, wire.EncodeZigZag(v.Int()))
+	case protoreflect.Uint64Kind:
+		b = wire.AppendVarint(b, v.Uint())
+	case protoreflect.Sfixed32Kind:
+		b = wire.AppendFixed32(b, uint32(v.Int()))
+	case protoreflect.Fixed32Kind:
+		b = wire.AppendFixed32(b, uint32(v.Uint()))
+	case protoreflect.FloatKind:
+		b = wire.AppendFixed32(b, math.Float32bits(float32(v.Float())))
+	case protoreflect.Sfixed64Kind:
+		b = wire.AppendFixed64(b, uint64(v.Int()))
+	case protoreflect.Fixed64Kind:
+		b = wire.AppendFixed64(b, v.Uint())
+	case protoreflect.DoubleKind:
+		b = wire.AppendFixed64(b, math.Float64bits(v.Float()))
+	case protoreflect.StringKind:
+		b = wire.AppendBytes(b, []byte(v.String()))
+	case protoreflect.BytesKind:
+		b = wire.AppendBytes(b, v.Bytes())
+	case protoreflect.MessageKind:
+		var pos int
+		var err error
+		b, pos = appendSpeculativeLength(b)
+		b, err = o.marshalMessage(b, v.Message())
+		if err != nil {
+			return nil, err
+		}
+		b = finishSpeculativeLength(b, pos)
+	case protoreflect.GroupKind:
+		var err error
+		b, err = o.marshalMessage(b, v.Message())
+		if err != nil {
+			return nil, err
+		}
+		b = wire.AppendVarint(b, wire.EncodeTag(num, wire.EndGroupType))
+	default:
+		return nil, errors.New("invalid kind %v", kind)
+	}
+	return b, nil
+}
diff --git a/proto/encode_test.go b/proto/encode_test.go
new file mode 100644
index 0000000..4c5034d
--- /dev/null
+++ b/proto/encode_test.go
@@ -0,0 +1,68 @@
+package proto_test
+
+import (
+	"bytes"
+	"fmt"
+	"reflect"
+	"testing"
+
+	protoV1 "github.com/golang/protobuf/proto"
+	//_ "github.com/golang/protobuf/v2/internal/legacy"
+	"github.com/golang/protobuf/v2/proto"
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestEncode(t *testing.T) {
+	for _, test := range testProtos {
+		for _, want := range test.decodeTo {
+			t.Run(fmt.Sprintf("%s (%T)", test.desc, want), func(t *testing.T) {
+				wire, err := proto.Marshal(want)
+				if err != nil {
+					t.Fatalf("Marshal error: %v\nMessage:\n%v", err, protoV1.MarshalTextString(want.(protoV1.Message)))
+				}
+
+				got := reflect.New(reflect.TypeOf(want).Elem()).Interface().(proto.Message)
+				if err := proto.Unmarshal(wire, got); err != nil {
+					t.Errorf("Unmarshal error: %v\nMessage:\n%v", err, protoV1.MarshalTextString(want.(protoV1.Message)))
+					return
+				}
+
+				if !protoV1.Equal(got.(protoV1.Message), want.(protoV1.Message)) {
+					t.Errorf("Unmarshal returned unexpected result; got:\n%v\nwant:\n%v", protoV1.MarshalTextString(got.(protoV1.Message)), protoV1.MarshalTextString(want.(protoV1.Message)))
+				}
+			})
+		}
+	}
+}
+
+func TestEncodeDeterministic(t *testing.T) {
+	for _, test := range testProtos {
+		for _, want := range test.decodeTo {
+			t.Run(fmt.Sprintf("%s (%T)", test.desc, want), func(t *testing.T) {
+				wire, err := proto.MarshalOptions{Deterministic: true}.Marshal(want)
+				if err != nil {
+					t.Fatalf("Marshal error: %v\nMessage:\n%v", err, protoV1.MarshalTextString(want.(protoV1.Message)))
+				}
+
+				wire2, err := proto.MarshalOptions{Deterministic: true}.Marshal(want)
+				if err != nil {
+					t.Fatalf("Marshal error: %v\nMessage:\n%v", err, protoV1.MarshalTextString(want.(protoV1.Message)))
+				}
+
+				if !bytes.Equal(wire, wire2) {
+					t.Fatalf("deterministic marshal returned varying results:\n%v", cmp.Diff(wire, wire2))
+				}
+
+				got := reflect.New(reflect.TypeOf(want).Elem()).Interface().(proto.Message)
+				if err := proto.Unmarshal(wire, got); err != nil {
+					t.Errorf("Unmarshal error: %v\nMessage:\n%v", err, protoV1.MarshalTextString(want.(protoV1.Message)))
+					return
+				}
+
+				if !protoV1.Equal(got.(protoV1.Message), want.(protoV1.Message)) {
+					t.Errorf("Unmarshal returned unexpected result; got:\n%v\nwant:\n%v", protoV1.MarshalTextString(got.(protoV1.Message)), protoV1.MarshalTextString(want.(protoV1.Message)))
+				}
+			})
+		}
+	}
+}