diff --git a/build.go b/build.go
index 2e38444..63a301f 100644
--- a/build.go
+++ b/build.go
@@ -36,10 +36,10 @@ func buildNode(v reflect.Value) *Node {
 		if name == "" {
 			name = strings.ToLower(strings.Join(camelCase(ft.Name), "-"))
 		}
-		help := ft.Tag.Get("help")
-		decoder, err := DecoderForField(ft)
-		if err != nil && ft.Type.Kind() != reflect.Struct {
-			panic(err)
+		decoder := DecoderForField(ft)
+		help, ok := ft.Tag.Lookup("help")
+		if !ok {
+			continue
 		}
 		dflt := ft.Tag.Get("default")
 		placeholder := ft.Tag.Get("placeholder")
@@ -53,11 +53,13 @@ func buildNode(v reflect.Value) *Node {
 		// group := ft.Tag.Get("group")
 		_, required := ft.Tag.Lookup("required")
 		_, optional := ft.Tag.Lookup("optional")
+		// Force field to be an argument, not a flag.
 		_, arg := ft.Tag.Lookup("arg")
 		env := ft.Tag.Get("env")
+		format := ft.Tag.Get("format")
 
-		// Nested structs are commands.
-		if ft.Type.Kind() == reflect.Struct {
+		// Nested structs are either commands or args.
+		if ft.Type.Kind() == reflect.Struct && decoder == nil {
 			child := buildNode(fv)
 			child.Help = help
 
@@ -65,8 +67,8 @@ func buildNode(v reflect.Value) *Node {
 			// a positional argument is provided to the child, and move it to the branching argument field.
 			if arg {
 				if len(child.Positional) == 0 {
-					panic(fmt.Errorf("positional branch %s.%s must have at least one child positional argument",
-						v.Type().Name(), ft.Name))
+					fail("positional branch %s.%s must have at least one child positional argument",
+						v.Type().Name(), ft.Name)
 				}
 				value := child.Positional[0]
 				child.Positional = child.Positional[1:]
@@ -83,6 +85,9 @@ func buildNode(v reflect.Value) *Node {
 				node.Children = append(node.Children, &Branch{Command: child})
 			}
 		} else {
+			if decoder == nil {
+				fail("no decoder for %s.%s (of type %s)", v.Type(), ft.Name, ft.Type)
+			}
 			value := Value{
 				Name:     name,
 				Help:     help,
@@ -90,6 +95,7 @@ func buildNode(v reflect.Value) *Node {
 				Value:    fv,
 				Field:    ft,
 				Required: !optional || required,
+				Format:   format,
 			}
 			if arg {
 				node.Positional = append(node.Positional, &value)
diff --git a/decoders.go b/decoders.go
index 88c5dbd..d1b2c10 100644
--- a/decoders.go
+++ b/decoders.go
@@ -5,6 +5,7 @@ import (
 	"reflect"
 	"strconv"
 	"strings"
+	"time"
 )
 
 type DecoderContext struct {
@@ -81,29 +82,34 @@ var _ NamedDecoder = &namedDecoder{}
 var (
 	namedDecoders = map[string]NamedDecoder{}
 	typeDecoders  = map[reflect.Type]TypeDecoder{}
-	kindDecoders  map[reflect.Kind]KindDecoder
+	kindDecoders  = map[reflect.Kind]KindDecoder{}
 )
 
 // DecoderForField finds a decoder for a struct field.
-func DecoderForField(field reflect.StructField) (Decoder, error) {
+//
+// Will return nil if a decoder can not be determined.
+func DecoderForField(field reflect.StructField) Decoder {
 	name, ok := field.Tag.Lookup("type")
 	if ok {
 		if decoder, ok := namedDecoders[name]; ok {
-			return decoder, nil
+			return decoder
 		}
 	}
 	return DecoderForType(field.Type)
 }
 
-func DecoderForType(typ reflect.Type) (Decoder, error) {
+// DecoderForType finds a decoder via a type or kind.
+//
+// Will return nil if a decoder can not be determined.
+func DecoderForType(typ reflect.Type) Decoder {
 	var decoder Decoder
 	var ok bool
 	if decoder, ok = typeDecoders[typ]; ok {
-		return decoder, nil
+		return decoder
 	} else if decoder, ok = kindDecoders[typ.Kind()]; ok {
-		return decoder, nil
+		return decoder
 	}
-	return nil, fmt.Errorf("no decoder for type %s", typ)
+	return nil
 }
 
 // RegisterDecoder registers decoders.
@@ -119,35 +125,59 @@ func RegisterDecoder(decoders ...Decoder) {
 		case NamedDecoder:
 			namedDecoders[decoder.Name()] = decoder
 		default:
-			panic("unsupported decoder type " + reflect.TypeOf(decoder).String())
+			fail("unsupported decoder type " + reflect.TypeOf(decoder).String())
 		}
 	}
 }
 
 func init() {
-	kindDecoders = map[reflect.Kind]KindDecoder{
-		reflect.Int:     NewKindDecoder(reflect.Int, intDecoder),
-		reflect.Int8:    NewKindDecoder(reflect.Int8, intDecoder),
-		reflect.Int16:   NewKindDecoder(reflect.Int16, intDecoder),
-		reflect.Int32:   NewKindDecoder(reflect.Int32, intDecoder),
-		reflect.Int64:   NewKindDecoder(reflect.Int64, intDecoder),
-		reflect.Uint:    NewKindDecoder(reflect.Uint, uintDecoder),
-		reflect.Uint8:   NewKindDecoder(reflect.Uint8, uintDecoder),
-		reflect.Uint16:  NewKindDecoder(reflect.Uint16, uintDecoder),
-		reflect.Uint32:  NewKindDecoder(reflect.Uint32, uintDecoder),
-		reflect.Uint64:  NewKindDecoder(reflect.Uint64, uintDecoder),
-		reflect.Float32: NewKindDecoder(reflect.Float32, floatDecoder(32)),
-		reflect.Float64: NewKindDecoder(reflect.Float64, floatDecoder(64)),
-		reflect.String: NewKindDecoder(reflect.String, func(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
+	RegisterDecoder(
+		NewKindDecoder(reflect.Int, intDecoder),
+		NewKindDecoder(reflect.Int8, intDecoder),
+		NewKindDecoder(reflect.Int16, intDecoder),
+		NewKindDecoder(reflect.Int32, intDecoder),
+		NewKindDecoder(reflect.Int64, intDecoder),
+		NewKindDecoder(reflect.Uint, uintDecoder),
+		NewKindDecoder(reflect.Uint8, uintDecoder),
+		NewKindDecoder(reflect.Uint16, uintDecoder),
+		NewKindDecoder(reflect.Uint32, uintDecoder),
+		NewKindDecoder(reflect.Uint64, uintDecoder),
+		NewKindDecoder(reflect.Float32, floatDecoder(32)),
+		NewKindDecoder(reflect.Float64, floatDecoder(64)),
+		NewKindDecoder(reflect.String, func(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
 			target.SetString(scan.PopValue("string"))
 			return nil
 		}),
-		reflect.Bool: NewKindDecoder(reflect.Bool, func(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
+		NewKindDecoder(reflect.Bool, func(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
 			target.SetBool(true)
 			return nil
 		}),
-		reflect.Slice: NewKindDecoder(reflect.Slice, sliceDecoder),
+		NewKindDecoder(reflect.Slice, sliceDecoder),
+		NewTypeDecoder(reflect.TypeOf(time.Time{}), timeDecoder),
+		NewTypeDecoder(reflect.TypeOf(time.Duration(0)), durationDecoder),
+	)
+}
+
+func durationDecoder(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
+	d, err := time.ParseDuration(scan.PopValue("duration"))
+	if err != nil {
+		return err
 	}
+	target.Set(reflect.ValueOf(d))
+	return nil
+}
+
+func timeDecoder(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
+	fmt := time.RFC3339
+	if ctx.Value.Format != "" {
+		fmt = ctx.Value.Format
+	}
+	t, err := time.Parse(fmt, scan.PopValue("time"))
+	if err != nil {
+		return err
+	}
+	target.Set(reflect.ValueOf(t))
+	return nil
 }
 
 func intDecoder(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
@@ -186,9 +216,9 @@ func sliceDecoder(ctx *DecoderContext, scan *Scanner, target reflect.Value) erro
 		sep = ","
 	}
 	childScanner := Scan(strings.Split(scan.PopValue("list"), sep)...)
-	childDecoder, err := DecoderForType(el)
-	if err != nil {
-		return err
+	childDecoder := DecoderForType(el)
+	if childDecoder == nil {
+		return fmt.Errorf("no decoder for element type of %s", target.Type())
 	}
 	for childScanner.Peek().Type != EOLToken {
 		childValue := reflect.New(el).Elem()
diff --git a/kong.go b/kong.go
index bc04265..c382b40 100644
--- a/kong.go
+++ b/kong.go
@@ -8,6 +8,16 @@ import (
 	"strings"
 )
 
+type Error struct {
+	msg string
+}
+
+func (e Error) Error() string { return e.msg }
+
+func fail(format string, args ...interface{}) {
+	panic(Error{fmt.Sprintf(format, args...)})
+}
+
 type Kong struct {
 	Model *Application
 	// Termination function (defaults to os.Exit)
@@ -35,7 +45,7 @@ func New(name, description string, ast interface{}) (*Kong, error) {
 func (k *Kong) Parse(args []string) (command string, err error) {
 	defer func() {
 		msg := recover()
-		if test, ok := msg.(TokenAssertionError); ok {
+		if test, ok := msg.(Error); ok {
 			err = test
 		} else if msg != nil {
 			panic(msg)
@@ -68,7 +78,8 @@ func (k *Kong) reset(node *Node) {
 	}
 }
 
-func (k *Kong) applyNode(scan *Scanner, node *Node) (command []string, err error) {
+func (k *Kong) applyNode(scan *Scanner, node *Node) (command []string, err error) { // nolint: gocyclo
+	positional := 0
 	for token := scan.Pop(); token.Type != EOLToken; token = scan.Pop() {
 		switch token.Type {
 		case UntypedToken:
@@ -98,6 +109,7 @@ func (k *Kong) applyNode(scan *Scanner, node *Node) (command []string, err error
 
 				// Short flag.
 			case strings.HasPrefix(token.Value, "-"):
+				// Note: tokens must be pushed in reverse order.
 				scan.PushTyped(token.Value[2:], ShortFlagTailToken)
 				scan.PushTyped(token.Value[1:2], ShortFlagToken)
 
@@ -106,6 +118,7 @@ func (k *Kong) applyNode(scan *Scanner, node *Node) (command []string, err error
 			}
 
 		case ShortFlagTailToken:
+			// Note: tokens must be pushed in reverse order.
 			scan.PushTyped(token.Value[1:], ShortFlagTailToken)
 			scan.PushTyped(token.Value[0:1], ShortFlagToken)
 
@@ -128,6 +141,19 @@ func (k *Kong) applyNode(scan *Scanner, node *Node) (command []string, err error
 
 		case PositionalArgumentToken:
 			scan.PushToken(token)
+			// Ensure we've consumed all positional arguments.
+			if positional < len(node.Positional) {
+				arg := node.Positional[positional]
+				err := arg.Decoder.Decode(&DecoderContext{Value: arg}, scan, arg.Value)
+				if err != nil {
+					return nil, err
+				}
+				command = append(command, "<"+arg.Name+">")
+				positional++
+				break
+			}
+
+			// After positional arguments have been consumed, handle commands and branching arguments.
 			for _, branch := range node.Children {
 				switch {
 				case branch.Command != nil:
@@ -165,7 +191,7 @@ func (k *Kong) applyNode(scan *Scanner, node *Node) (command []string, err error
 func matchFlags(flags []*Flag, token Token, scan *Scanner, matcher func(f *Flag) bool) (err error) {
 	defer func() {
 		msg := recover()
-		if test, ok := msg.(TokenAssertionError); ok {
+		if test, ok := msg.(Error); ok {
 			err = fmt.Errorf("%s %s", token, test)
 		} else if msg != nil {
 			panic(msg)
diff --git a/kong_test.go b/kong_test.go
index 37d40de..2f3d96b 100644
--- a/kong_test.go
+++ b/kong_test.go
@@ -13,7 +13,23 @@ func mustNew(t *testing.T, cli interface{}) *Kong {
 	return parser
 }
 
-func TestArgument(t *testing.T) {
+func TestArgumentSequence(t *testing.T) {
+	var cli struct {
+		User struct {
+			Create struct {
+				ID    int    `arg:"" help:""`
+				First string `arg:"" help:""`
+				Last  string `arg:"" help:""`
+			} `help:""`
+		} `help:""`
+	}
+	p := mustNew(t, &cli)
+	cmd, err := p.Parse([]string{"user", "create", "10", "Alec", "Thomas"})
+	require.NoError(t, err)
+	require.Equal(t, "user create <id> <first> <last>", cmd)
+}
+
+func TestBranchingArgument(t *testing.T) {
 	/*
 		app user create <id> <first> <last>
 		app	user <id> delete
@@ -21,33 +37,35 @@ func TestArgument(t *testing.T) {
 
 	*/
 	var cli struct {
-		Create struct {
-			Id    string `arg:"true"`
-			First string `arg:"true"`
-			Last  string `arg:"true"`
-		}
+		User struct {
+			Create struct {
+				ID    string `arg:"" help:""`
+				First string `arg:"" help:""`
+				Last  string `arg:"" help:""`
+			} `help:""`
 
-		// Branching argument.
-		Id struct {
-			Id     int `arg:"true"`
-			Flag   int
-			Delete struct{}
-			Rename struct {
-				To string
-			}
-		} `arg:"true"`
+			// Branching argument.
+			ID struct {
+				ID     int      `arg:"" help:""`
+				Flag   int      `help:""`
+				Delete struct{} `help:""`
+				Rename struct {
+					To string
+				} `help:""`
+			} `arg:"" help:""`
+		} `help:"Manage users."`
 	}
 	p := mustNew(t, &cli)
-	cmd, err := p.Parse([]string{"10", "delete"})
+	cmd, err := p.Parse([]string{"user", "10", "delete"})
 	require.NoError(t, err)
-	require.Equal(t, 10, cli.Id.Id)
-	require.Equal(t, "<id> delete", cmd)
+	require.Equal(t, 10, cli.User.ID.ID)
+	require.Equal(t, "user <id> delete", cmd)
 }
 
 func TestResetWithDefaults(t *testing.T) {
 	var cli struct {
-		Flag            string
-		FlagWithDefault string `default:"default"`
+		Flag            string `help:""`
+		FlagWithDefault string `default:"default" help:""`
 	}
 	cli.Flag = "BLAH"
 	cli.FlagWithDefault = "BLAH"
@@ -60,10 +78,17 @@ func TestResetWithDefaults(t *testing.T) {
 
 func TestSlice(t *testing.T) {
 	var cli struct {
-		Slice []int
+		Slice []int `help:""`
 	}
 	parser := mustNew(t, &cli)
 	_, err := parser.Parse([]string{"--slice=1,2,3"})
 	require.NoError(t, err)
 	require.Equal(t, []int{1, 2, 3}, cli.Slice)
 }
+
+func TestUnsupportedfieldErrors(t *testing.T) {
+	var cli struct {
+		Keys map[string]string `help:""`
+	}
+	require.Panics(t, func() { mustNew(t, &cli) })
+}
diff --git a/model.go b/model.go
index 3ec5e0f..51c12d9 100644
--- a/model.go
+++ b/model.go
@@ -27,6 +27,7 @@ type Value struct {
 	Field    reflect.StructField
 	Value    reflect.Value
 	Required bool
+	Format   string // Formatting directive, if applicable.
 }
 
 type Positional = Value
diff --git a/scanner.go b/scanner.go
index 78c5b3b..e93b717 100644
--- a/scanner.go
+++ b/scanner.go
@@ -1,7 +1,6 @@
 package kong
 
 import (
-	"fmt"
 	"strconv"
 )
 
@@ -19,12 +18,6 @@ const (
 	PositionalArgumentToken // <arg>
 )
 
-type TokenAssertionError struct{ err error }
-
-func (t TokenAssertionError) Error() string {
-	return t.err.Error()
-}
-
 type Token struct {
 	Value string
 	Type  TokenType
@@ -84,11 +77,11 @@ func (s *Scanner) Pop() Token {
 	return arg
 }
 
-// PopValue or panic with TokenAssertionError.
+// PopValue or panic with Error.
 func (s *Scanner) PopValue(context string) string {
 	t := s.Pop()
 	if !t.IsValue() {
-		panic(TokenAssertionError{fmt.Errorf("expected %s value but got %s", context, t)})
+		fail("expected %s value but got %s", context, t)
 	}
 	return t.Value
 }