diff --git a/genmethod.go b/genmethod.go
index 2c99e77..f28d44c 100644
--- a/genmethod.go
+++ b/genmethod.go
@@ -182,6 +182,9 @@ func (m *marshalMethod) receiver() Receiver {
 func (m *marshalMethod) intermediateType(name string) Struct {
 	s := Struct{Name: name}
 	for _, f := range m.mtyp.Fields {
+		if m.isUnmarshal && f.function != nil {
+			continue // fields generated from functions cannot be assigned on unmarshal
+		}
 		typ := f.typ
 		if m.isUnmarshal {
 			typ = ensureNilCheckable(typ)
@@ -197,6 +200,10 @@ func (m *marshalMethod) intermediateType(name string) Struct {
 
 func (m *marshalMethod) unmarshalConversions(from, to Var, format string) (s []Statement) {
 	for _, f := range m.mtyp.Fields {
+		if f.function != nil {
+			continue // fields generated from functions cannot be assigned
+		}
+
 		accessFrom := Dotted{Receiver: from, Name: f.name}
 		accessTo := Dotted{Receiver: to, Name: f.name}
 		typ := ensureNilCheckable(f.typ)
@@ -231,8 +238,11 @@ func (m *marshalMethod) marshalConversions(from, to Var, format string) (s []Sta
 	for _, f := range m.mtyp.Fields {
 		accessFrom := Dotted{Receiver: from, Name: f.name}
 		accessTo := Dotted{Receiver: to, Name: f.name}
-		conversion := m.convert(accessFrom, accessTo, f.origTyp, f.typ)
-		s = append(s, conversion...)
+		if f.function != nil {
+			s = append(s, m.convert(CallFunction{Func: accessFrom}, accessTo, f.origTyp, f.typ)...)
+		} else {
+			s = append(s, m.convert(accessFrom, accessTo, f.origTyp, f.typ)...)
+		}
 	}
 	return s
 }
diff --git a/internal/tests/funcoverride/input.go b/internal/tests/funcoverride/input.go
new file mode 100644
index 0000000..6edef47
--- /dev/null
+++ b/internal/tests/funcoverride/input.go
@@ -0,0 +1,33 @@
+// Copyright 2017 Felix Lange <fjl@twurst.com>.
+// Use of this source code is governed by the MIT license,
+// which can be found in the LICENSE file.
+
+//go:generate gencodec -type Z -field-override Zo -formats json,yaml,toml -out output.go
+
+package funcoverride
+
+import (
+	"fmt"
+)
+
+type Z struct {
+	S string `json:"s"`
+	I int32  `json:"iVal"`
+}
+
+func (z *Z) Hash() string {
+	return fmt.Sprintf("%s-%d", z.S, z.I)
+}
+
+func (z *Z) MultiplyIByTwo() int32 {
+	return 2 * z.I
+}
+
+func (z *Z) NotUsed() string {
+	return "not used"
+}
+
+type Zo struct {
+	Hash           string
+	MultiplyIByTwo int64 `json:"multipliedByTwo"`
+}
diff --git a/internal/tests/funcoverride/input_test.go b/internal/tests/funcoverride/input_test.go
new file mode 100644
index 0000000..325b3c6
--- /dev/null
+++ b/internal/tests/funcoverride/input_test.go
@@ -0,0 +1,44 @@
+// Copyright 2017 Felix Lange <fjl@twurst.com>.
+// Use of this source code is governed by the MIT license,
+// which can be found in the LICENSE file.
+
+package funcoverride
+
+import (
+	"encoding/json"
+	"fmt"
+	"testing"
+)
+
+func TestOverrideFuncJSON(t *testing.T) {
+	z := Z{"str", 1234}
+	hash := z.Hash()
+	multiply := z.MultiplyIByTwo()
+	want := fmt.Sprintf(`{"s":"%s","iVal":%d,"Hash":"%s","multipliedByTwo":%d}`, z.S, z.I, hash, multiply)
+	out, err := json.Marshal(z)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(out) != want {
+		t.Fatalf("got %#q, want %#q", string(out), want)
+	}
+
+	var zUnmarshaled Z
+	if err := json.Unmarshal([]byte(want), &zUnmarshaled); err != nil {
+		t.Fatalf("could not unmarshal Z: %v", err)
+	}
+	if zUnmarshaled.I != z.I {
+		t.Errorf("Z.I has an unexpected value, want %d, got %d", z.I, zUnmarshaled.I)
+	}
+	if zUnmarshaled.S != z.S {
+		t.Errorf("Z.Str has an unexpected value, want %s, got %s", z.S, zUnmarshaled.S)
+	}
+	uHash := zUnmarshaled.Hash()
+	if uHash != hash {
+		t.Errorf("Z.Hash() returned unexpected value, want %s, got %s", hash, uHash)
+	}
+	uMultiply := zUnmarshaled.MultiplyIByTwo()
+	if uMultiply != multiply {
+		t.Errorf("Z.MultiplIByTwo() returned unexpected value, want %d, got %d", multiply, uMultiply)
+	}
+}
diff --git a/internal/tests/funcoverride/output.go b/internal/tests/funcoverride/output.go
new file mode 100644
index 0000000..cb9ad80
--- /dev/null
+++ b/internal/tests/funcoverride/output.go
@@ -0,0 +1,106 @@
+// Code generated by github.com/fjl/gencodec. DO NOT EDIT.
+
+package funcoverride
+
+import (
+	"encoding/json"
+)
+
+func (z Z) MarshalJSON() ([]byte, error) {
+	type Z struct {
+		S              string `json:"s"`
+		I              int32  `json:"iVal"`
+		Hash           string
+		MultiplyIByTwo int64 `json:"multipliedByTwo"`
+	}
+	var enc Z
+	enc.S = z.S
+	enc.I = z.I
+	enc.Hash = z.Hash()
+	enc.MultiplyIByTwo = int64(z.MultiplyIByTwo())
+	return json.Marshal(&enc)
+}
+
+func (z *Z) UnmarshalJSON(input []byte) error {
+	type Z struct {
+		S *string `json:"s"`
+		I *int32  `json:"iVal"`
+	}
+	var dec Z
+	if err := json.Unmarshal(input, &dec); err != nil {
+		return err
+	}
+	if dec.S != nil {
+		z.S = *dec.S
+	}
+	if dec.I != nil {
+		z.I = *dec.I
+	}
+	return nil
+}
+
+func (z Z) MarshalYAML() (interface{}, error) {
+	type Z struct {
+		S              string `json:"s"`
+		I              int32  `json:"iVal"`
+		Hash           string
+		MultiplyIByTwo int64 `json:"multipliedByTwo"`
+	}
+	var enc Z
+	enc.S = z.S
+	enc.I = z.I
+	enc.Hash = z.Hash()
+	enc.MultiplyIByTwo = int64(z.MultiplyIByTwo())
+	return &enc, nil
+}
+
+func (z *Z) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	type Z struct {
+		S *string `json:"s"`
+		I *int32  `json:"iVal"`
+	}
+	var dec Z
+	if err := unmarshal(&dec); err != nil {
+		return err
+	}
+	if dec.S != nil {
+		z.S = *dec.S
+	}
+	if dec.I != nil {
+		z.I = *dec.I
+	}
+	return nil
+}
+
+func (z Z) MarshalTOML() (interface{}, error) {
+	type Z struct {
+		S              string `json:"s"`
+		I              int32  `json:"iVal"`
+		Hash           string
+		MultiplyIByTwo int64 `json:"multipliedByTwo"`
+	}
+	var enc Z
+	enc.S = z.S
+	enc.I = z.I
+	enc.Hash = z.Hash()
+	enc.MultiplyIByTwo = int64(z.MultiplyIByTwo())
+	return &enc, nil
+}
+
+func (z *Z) UnmarshalTOML(unmarshal func(interface{}) error) error {
+	type Z struct {
+		S *string `json:"s"`
+		I *int32  `json:"iVal"`
+	}
+	var dec Z
+	if err := unmarshal(&dec); err != nil {
+		return err
+	}
+	if dec.S != nil {
+		z.S = *dec.S
+	}
+	if dec.I != nil {
+		z.I = *dec.I
+	}
+	return nil
+}
diff --git a/internal/tests/mapconv/input.go b/internal/tests/mapconv/input.go
index 89d4d49..c9db943 100644
--- a/internal/tests/mapconv/input.go
+++ b/internal/tests/mapconv/input.go
@@ -21,8 +21,14 @@ type X struct {
 	NoConvNamed map[string]int
 }
 
+func (x *X) Func() map[string]int {
+	return map[string]int{"a": 1, "b": 2}
+}
+
 type Xo struct {
 	Map         map[replacedString]replacedInt
 	Named       namedMap2
 	NoConvNamed namedMap
+
+	Func        map[replacedString]replacedInt
 }
diff --git a/internal/tests/mapconv/output.go b/internal/tests/mapconv/output.go
index 3188583..4593155 100644
--- a/internal/tests/mapconv/output.go
+++ b/internal/tests/mapconv/output.go
@@ -12,6 +12,7 @@ func (x X) MarshalJSON() ([]byte, error) {
 		Named       namedMap2
 		NoConv      map[string]int
 		NoConvNamed namedMap
+		Func        map[replacedString]replacedInt
 	}
 	var enc X
 	if x.Map != nil {
@@ -28,6 +29,12 @@ func (x X) MarshalJSON() ([]byte, error) {
 	}
 	enc.NoConv = x.NoConv
 	enc.NoConvNamed = x.NoConvNamed
+	if x.Func() != nil {
+		enc.Func = make(map[replacedString]replacedInt, len(x.Func()))
+		for k, v := range x.Func() {
+			enc.Func[replacedString(k)] = replacedInt(v)
+		}
+	}
 	return json.Marshal(&enc)
 }
 
@@ -69,6 +76,7 @@ func (x X) MarshalYAML() (interface{}, error) {
 		Named       namedMap2
 		NoConv      map[string]int
 		NoConvNamed namedMap
+		Func        map[replacedString]replacedInt
 	}
 	var enc X
 	if x.Map != nil {
@@ -85,6 +93,12 @@ func (x X) MarshalYAML() (interface{}, error) {
 	}
 	enc.NoConv = x.NoConv
 	enc.NoConvNamed = x.NoConvNamed
+	if x.Func() != nil {
+		enc.Func = make(map[replacedString]replacedInt, len(x.Func()))
+		for k, v := range x.Func() {
+			enc.Func[replacedString(k)] = replacedInt(v)
+		}
+	}
 	return &enc, nil
 }
 
@@ -126,6 +140,7 @@ func (x X) MarshalTOML() (interface{}, error) {
 		Named       namedMap2
 		NoConv      map[string]int
 		NoConvNamed namedMap
+		Func        map[replacedString]replacedInt
 	}
 	var enc X
 	if x.Map != nil {
@@ -142,6 +157,12 @@ func (x X) MarshalTOML() (interface{}, error) {
 	}
 	enc.NoConv = x.NoConv
 	enc.NoConvNamed = x.NoConvNamed
+	if x.Func() != nil {
+		enc.Func = make(map[replacedString]replacedInt, len(x.Func()))
+		for k, v := range x.Func() {
+			enc.Func[replacedString(k)] = replacedInt(v)
+		}
+	}
 	return &enc, nil
 }
 
diff --git a/internal/tests/sliceconv/input.go b/internal/tests/sliceconv/input.go
index 7b59f4d..1d020d6 100644
--- a/internal/tests/sliceconv/input.go
+++ b/internal/tests/sliceconv/input.go
@@ -20,9 +20,15 @@ type X struct {
 	NoConvNamed []int
 }
 
+func (x *X) Func() []int {
+	return []int{1,2,3,4}
+}
+
 type Xo struct {
 	Slice       []replacedInt
 	Named       namedSlice2
 	ByteString  []byte
 	NoConvNamed namedSlice
+
+	Func []replacedInt
 }
diff --git a/internal/tests/sliceconv/output.go b/internal/tests/sliceconv/output.go
index 335881e..fd7d80a 100644
--- a/internal/tests/sliceconv/output.go
+++ b/internal/tests/sliceconv/output.go
@@ -13,6 +13,7 @@ func (x X) MarshalJSON() ([]byte, error) {
 		ByteString  []byte
 		NoConv      []int
 		NoConvNamed namedSlice
+		Func        []replacedInt
 	}
 	var enc X
 	if x.Slice != nil {
@@ -30,6 +31,12 @@ func (x X) MarshalJSON() ([]byte, error) {
 	enc.ByteString = []byte(x.ByteString)
 	enc.NoConv = x.NoConv
 	enc.NoConvNamed = x.NoConvNamed
+	if x.Func() != nil {
+		enc.Func = make([]replacedInt, len(x.Func()))
+		for k, v := range x.Func() {
+			enc.Func[k] = replacedInt(v)
+		}
+	}
 	return json.Marshal(&enc)
 }
 
@@ -76,6 +83,7 @@ func (x X) MarshalYAML() (interface{}, error) {
 		ByteString  []byte
 		NoConv      []int
 		NoConvNamed namedSlice
+		Func        []replacedInt
 	}
 	var enc X
 	if x.Slice != nil {
@@ -93,6 +101,12 @@ func (x X) MarshalYAML() (interface{}, error) {
 	enc.ByteString = []byte(x.ByteString)
 	enc.NoConv = x.NoConv
 	enc.NoConvNamed = x.NoConvNamed
+	if x.Func() != nil {
+		enc.Func = make([]replacedInt, len(x.Func()))
+		for k, v := range x.Func() {
+			enc.Func[k] = replacedInt(v)
+		}
+	}
 	return &enc, nil
 }
 
@@ -139,6 +153,7 @@ func (x X) MarshalTOML() (interface{}, error) {
 		ByteString  []byte
 		NoConv      []int
 		NoConvNamed namedSlice
+		Func        []replacedInt
 	}
 	var enc X
 	if x.Slice != nil {
@@ -156,6 +171,12 @@ func (x X) MarshalTOML() (interface{}, error) {
 	enc.ByteString = []byte(x.ByteString)
 	enc.NoConv = x.NoConv
 	enc.NoConvNamed = x.NoConvNamed
+	if x.Func() != nil {
+		enc.Func = make([]replacedInt, len(x.Func()))
+		for k, v := range x.Func() {
+			enc.Func[k] = replacedInt(v)
+		}
+	}
 	return &enc, nil
 }
 
diff --git a/main.go b/main.go
index 9885bad..ee06597 100644
--- a/main.go
+++ b/main.go
@@ -32,11 +32,17 @@ Field Type Overrides
 An invocation of gencodec can specify an additional 'field override' struct from which
 marshaling type replacements are taken. If the override struct contains a field whose name
 matches the original type, the generated marshaling methods will use the overridden type
-and convert to and from the original field type.
+and convert to and from the original field type. If the override struct contains a field F
+of type T, which does not exist in the original type, and the original type has a method
+named F with no arguments and return type assignable to T, the method is called by Marshal*.
+If there is a matching method F but the return type or arguments are unsuitable, an error
+is raised.
 
 In this example, the specialString type implements json.Unmarshaler to enforce additional
 parsing rules. When json.Unmarshal is used with type foo, the specialString unmarshaler
-will be used to parse the value of SpecialField.
+will be used to parse the value of SpecialField. The result of foo.Func() is added to the
+result on marshaling under the key `id`. If the input on unmarshal contains a key `id` this
+field is ignored.
 
 	//go:generate gencodec -type foo -field-override fooMarshaling -out foo_json.go
 
@@ -45,8 +51,13 @@ will be used to parse the value of SpecialField.
 		SpecialField string
 	}
 
+	func (f foo) Func() string {
+	    return f.Field + "-" + f.SpecialField
+	}
+
 	type fooMarshaling struct {
 		SpecialField specialString // overrides type of SpecialField when marshaling/unmarshaling
+		Func string `json:"id"`    // adds the result of foo.Func() to the serialised object under the key id
 	}
 
 Relaxed Field Conversions
@@ -186,7 +197,8 @@ func (cfg *Config) process() (code []byte, err error) {
 		if err != nil {
 			return nil, fmt.Errorf("can't find field replacement type %s: %v", cfg.FieldOverride, err)
 		}
-		err = mtyp.loadOverrides(cfg.FieldOverride, otyp.Underlying().(*types.Struct))
+
+		err = mtyp.loadOverrides(typ, otyp.Underlying().(*types.Struct))
 		if err != nil {
 			return nil, err
 		}
@@ -266,10 +278,11 @@ type marshalerType struct {
 
 // marshalerField represents a field of the intermediate marshaling type.
 type marshalerField struct {
-	name    string
-	typ     types.Type
-	origTyp types.Type
-	tag     string
+	name     string
+	typ      types.Type
+	origTyp  types.Type
+	tag      string
+	function *types.Func // map to a function instead of a field
 }
 
 func newMarshalerType(fs *token.FileSet, imp types.Importer, typ *types.Named) *marshalerType {
@@ -287,24 +300,46 @@ func newMarshalerType(fs *token.FileSet, imp types.Importer, typ *types.Named) *
 		if !f.Exported() {
 			continue
 		}
+		if f.Anonymous() {
+			fmt.Fprintf(os.Stderr, "Warning: ignoring embedded field %s\n", f.Name())
+			continue
+		}
+
 		mf := &marshalerField{
 			name:    f.Name(),
 			typ:     f.Type(),
 			origTyp: f.Type(),
 			tag:     styp.Tag(i),
 		}
-		if f.Anonymous() {
-			fmt.Fprintf(os.Stderr, "Warning: ignoring embedded field %s\n", f.Name())
-			continue
-		}
+
 		mtyp.Fields = append(mtyp.Fields, mf)
 	}
+
 	return mtyp
 }
 
+// findFunction returns a function with `name` that accepts no arguments
+// and returns a single value that is convertible to the given to type.
+func findFunction(typ *types.Named, name string, to types.Type) (*types.Func, types.Type) {
+	for i := 0; i < typ.NumMethods(); i++ {
+		fun := typ.Method(i)
+		if fun.Name() != name || !fun.Exported() {
+			continue
+		}
+		sign := fun.Type().(*types.Signature)
+		if sign.Params().Len() != 0 || sign.Results().Len() != 1 {
+			continue
+		}
+		if err := checkConvertible(sign.Results().At(0).Type(), to); err == nil {
+			return fun, sign.Results().At(0).Type()
+		}
+	}
+	return nil, nil
+}
+
 // loadOverrides sets field types of the intermediate marshaling type from
 // matching fields of otyp.
-func (mtyp *marshalerType) loadOverrides(otypename string, otyp *types.Struct) error {
+func (mtyp *marshalerType) loadOverrides(typ *types.Named, otyp *types.Struct) error {
 	for i := 0; i < otyp.NumFields(); i++ {
 		of := otyp.Field(i)
 		if of.Anonymous() || !of.Exported() {
@@ -312,7 +347,13 @@ func (mtyp *marshalerType) loadOverrides(otypename string, otyp *types.Struct) e
 		}
 		f := mtyp.fieldByName(of.Name())
 		if f == nil {
-			return fmt.Errorf("%v: no matching field for %s in original type %s", mtyp.fs.Position(of.Pos()), of.Name(), mtyp.name)
+			// field not defined in original type, check if it maps to a suitable function and add it as an override
+			if fun, retType := findFunction(typ, of.Name(), of.Type()); fun != nil {
+				f = &marshalerField{name: fun.Name(), origTyp: retType, typ: of.Type(), function: fun, tag: otyp.Tag(i)}
+				mtyp.Fields = append(mtyp.Fields, f)
+			} else {
+				return fmt.Errorf("%v: no matching field or function for %s in original type %s", mtyp.fs.Position(of.Pos()), of.Name(), mtyp.name)
+			}
 		}
 		if err := checkConvertible(of.Type(), f.origTyp); err != nil {
 			return fmt.Errorf("%v: invalid field override: %v", mtyp.fs.Position(of.Pos()), err)
diff --git a/main_test.go b/main_test.go
index 608ac60..7c196a9 100644
--- a/main_test.go
+++ b/main_test.go
@@ -24,6 +24,7 @@ func TestGolden(t *testing.T) {
 		Config{Dir: "nameclash", Type: "Y", FieldOverride: "Yo", Formats: AllFormats},
 		Config{Dir: "omitempty", Type: "X", FieldOverride: "Xo", Formats: AllFormats},
 		Config{Dir: "reqfield", Type: "X", Formats: []string{"json"}},
+		Config{Dir: "funcoverride", Type: "Z", FieldOverride: "Zo", Formats: AllFormats},
 	}
 	for _, test := range tests {
 		test := test