diff --git a/help.go b/help.go
index 4f92437..8fe9cc7 100644
--- a/help.go
+++ b/help.go
@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"go/doc"
 	"io"
-	"reflect"
 	"strings"
 )
 
@@ -197,8 +196,5 @@ func formatFlag(haveShort bool, flag *Flag) string {
 	if !isBool {
 		flagString += fmt.Sprintf("=%s", flag.FormatPlaceHolder())
 	}
-	if flag.Value.Target.Kind() == reflect.Slice {
-		flagString += " ..."
-	}
 	return flagString
 }
diff --git a/help_test.go b/help_test.go
index 5d5e76d..9aa6004 100644
--- a/help_test.go
+++ b/help_test.go
@@ -10,9 +10,11 @@ import (
 func TestHelp(t *testing.T) {
 	// nolint: govet
 	var cli struct {
-		String   string `help:"A string flag."`
-		Bool     bool   `help:"A bool flag with very long help that wraps a lot and is verbose and is really verbose."`
-		Required bool   `required help:"A required flag."`
+		String   string         `help:"A string flag."`
+		Bool     bool           `help:"A bool flag with very long help that wraps a lot and is verbose and is really verbose."`
+		Slice    []string       `help:"A slice of strings." placeholder:"STR"`
+		Map      map[string]int `help:"A map of strings to ints."`
+		Required bool           `required help:"A required flag."`
 
 		One struct {
 			Flag string `help:"Nested flag."`
@@ -60,6 +62,8 @@ Flags:
   --string=STRING  A string flag.
   --bool           A bool flag with very long help that wraps a lot and is
                    verbose and is really verbose.
+  --slice=STR,...  A slice of strings.
+  --map=KEY=VALUE  A map of strings to ints.
   --required       A required flag.
 
 Commands:
@@ -91,6 +95,8 @@ Flags:
   --string=STRING   A string flag.
   --bool            A bool flag with very long help that wraps a lot and is
                     verbose and is really verbose.
+  --slice=STR,...   A slice of strings.
+  --map=KEY=VALUE   A map of strings to ints.
   --required        A required flag.
 
   --flag=STRING     Nested flag under two.
diff --git a/kong_test.go b/kong_test.go
index bb3d322..2eaaf72 100644
--- a/kong_test.go
+++ b/kong_test.go
@@ -139,7 +139,7 @@ func TestArgSliceWithSeparator(t *testing.T) {
 
 func TestUnsupportedFieldErrors(t *testing.T) {
 	var cli struct {
-		Keys map[string]string
+		Keys struct{}
 	}
 	_, err := New(&cli)
 	require.Error(t, err)
@@ -401,3 +401,12 @@ func TestDuplicateSliceAccumulates(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, []int{1, 2, 3, 4}, cli.Flag)
 }
+
+func TestMapFlag(t *testing.T) {
+	var cli struct {
+		Set map[string]int
+	}
+	_, err := mustNew(t, &cli).Parse([]string{"--set", "a=10", "--set", "b=20"})
+	require.NoError(t, err)
+	require.Equal(t, map[string]int{"a": 10, "b": 20}, cli.Set)
+}
diff --git a/mapper.go b/mapper.go
index 6b4d05a..dcbacd0 100644
--- a/mapper.go
+++ b/mapper.go
@@ -153,7 +153,8 @@ func (d *Registry) RegisterDefaults() *Registry {
 		RegisterKind(reflect.Bool, boolMapper{}).
 		RegisterType(reflect.TypeOf(time.Time{}), timeDecoder()).
 		RegisterType(reflect.TypeOf(time.Duration(0)), durationDecoder()).
-		RegisterKind(reflect.Slice, sliceDecoder(d))
+		RegisterKind(reflect.Slice, sliceDecoder(d)).
+		RegisterKind(reflect.Map, mapDecoder(d))
 }
 
 type boolMapper struct{}
@@ -228,6 +229,36 @@ func floatDecoder(bits int) MapperFunc {
 
 func mapDecoder(d *Registry) MapperFunc {
 	return func(ctx *DecodeContext, target reflect.Value) error {
+		if target.IsNil() {
+			target.Set(reflect.MakeMap(target.Type()))
+		}
+		el := target.Type()
+		sep := ctx.Value.Tag.Sep
+		if sep == 0 {
+			sep = '='
+		}
+		token := ctx.Scan.PopValue("map")
+		parts := SplitEscaped(token, sep)
+		if len(parts) != 2 {
+			return fmt.Errorf("expected \"<key>%c<value>\" but got %q", sep, token)
+		}
+		key, value := parts[0], parts[1]
+
+		keyScanner := Scan(key)
+		keyDecoder := d.ForType(el.Key())
+		keyValue := reflect.New(el.Key()).Elem()
+		if err := keyDecoder.Decode(ctx.WithScanner(keyScanner), keyValue); err != nil {
+			return fmt.Errorf("invalid map key %q", key)
+		}
+
+		valueScanner := Scan(value)
+		valueDecoder := d.ForType(el.Elem())
+		valueValue := reflect.New(el.Elem()).Elem()
+		if err := valueDecoder.Decode(ctx.WithScanner(valueScanner), valueValue); err != nil {
+			return fmt.Errorf("invalid map value %q", value)
+		}
+
+		target.SetMapIndex(keyValue, valueValue)
 		return nil
 	}
 }
@@ -236,12 +267,15 @@ func sliceDecoder(d *Registry) MapperFunc {
 	return func(ctx *DecodeContext, target reflect.Value) error {
 		el := target.Type().Elem()
 		sep := ctx.Value.Tag.Sep
+		if sep == 0 {
+			sep = ','
+		}
 		var childScanner *Scanner
 		if ctx.Value.Flag != nil {
 			// If decoding a flag, we need an argument.
 			childScanner = Scan(SplitEscaped(ctx.Scan.PopValue("list"), sep)...)
 		} else {
-			tokens := ctx.Scan.PopUntil(func(t Token) bool { return !t.IsValue() })
+			tokens := ctx.Scan.PopWhile(func(t Token) bool { return t.IsValue() })
 			childScanner = Scan(tokens...)
 		}
 		childDecoder := d.ForType(el)
diff --git a/model.go b/model.go
index a7834ca..cdf5e50 100644
--- a/model.go
+++ b/model.go
@@ -164,11 +164,21 @@ func (v *Value) Summary() string {
 	return argText
 }
 
-// IsCumulative returns true of the value is a slice.
+// IsCumulative returns true if the type can be accumulated into.
 func (v *Value) IsCumulative() bool {
+	return v.IsSlice() || v.IsMap()
+}
+
+// IsSlice returns true if the value is a slice.
+func (v *Value) IsSlice() bool {
 	return v.Target.Kind() == reflect.Slice
 }
 
+// IsMap returns true if the value is a map.
+func (v *Value) IsMap() bool {
+	return v.Target.Kind() == reflect.Map
+}
+
 // IsBool returns true if the underlying value is a boolean.
 func (v *Value) IsBool() bool {
 	if m, ok := v.Mapper.(BoolMapper); ok && m.IsBool() {
@@ -229,8 +239,8 @@ func (f *Flag) String() string {
 // FormatPlaceHolder formats the placeholder string for a Flag.
 func (f *Flag) FormatPlaceHolder() string {
 	tail := ""
-	if f.Value.IsCumulative() {
-		tail += ", ..."
+	if f.Value.IsSlice() {
+		tail += ",..."
 	}
 	if f.Default != "" {
 		if f.Value.Target.Kind() == reflect.String {
@@ -241,5 +251,8 @@ func (f *Flag) FormatPlaceHolder() string {
 	if f.PlaceHolder != "" {
 		return f.PlaceHolder + tail
 	}
+	if f.Value.IsMap() {
+		return "KEY=VALUE" + tail
+	}
 	return strings.ToUpper(f.Name) + tail
 }
diff --git a/resolver.go b/resolver.go
index 08520ae..fd5a02d 100755
--- a/resolver.go
+++ b/resolver.go
@@ -26,7 +26,11 @@ func JSON(r io.Reader) (ResolverFunc, error) {
 		if !ok {
 			return "", nil
 		}
-		value, err := jsonDecodeValue(flag.Tag.Sep, raw)
+		sep := flag.Tag.Sep
+		if sep == 0 {
+			sep = ','
+		}
+		value, err := jsonDecodeValue(sep, raw)
 		if err != nil {
 			return "", err
 		}
diff --git a/tag.go b/tag.go
index 17a71e6..1573806 100644
--- a/tag.go
+++ b/tag.go
@@ -130,14 +130,6 @@ func parseTag(fv reflect.Value, ft reflect.StructField) *Tag {
 	t.Hidden = t.Has("hidden")
 	t.Format, _ = t.Get("format")
 	t.Sep, _ = t.GetRune("sep")
-	if t.Sep == 0 {
-		if t.Cmd || t.Arg {
-			t.Sep = ' '
-		} else {
-			t.Sep = ','
-		}
-	}
-
 	t.PlaceHolder, _ = t.Get("placeholder")
 	if t.PlaceHolder == "" {
 		t.PlaceHolder = strings.ToUpper(dashedString(fv.Type().Name()))