diff --git a/bool.go b/bool.go
index 010576c..70e2e0a 100644
--- a/bool.go
+++ b/bool.go
@@ -5,6 +5,13 @@
 	"strconv"
 )
 
+// optional interface to indicate boolean flags that can be
+// supplied without "=value" text
+type boolFlag interface {
+	Value
+	IsBoolFlag() bool
+}
+
 // -- bool Value
 type boolValue bool
 
@@ -25,6 +32,8 @@
 
 func (b *boolValue) String() string { return fmt.Sprintf("%v", *b) }
 
+func (b *boolValue) IsBoolFlag() bool { return true }
+
 // BoolVar defines a bool flag with specified name, default value, and usage string.
 // The argument p points to a bool variable in which to store the value of the flag.
 func (f *FlagSet) BoolVar(p *bool, name string, value bool, usage string) {
diff --git a/bool_test.go b/bool_test.go
new file mode 100644
index 0000000..72a12be
--- /dev/null
+++ b/bool_test.go
@@ -0,0 +1,163 @@
+// Copyright 2009 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 pflag_test
+
+import (
+	"fmt"
+	"strconv"
+	"testing"
+
+	. "github.com/ogier/pflag"
+)
+
+// This value can be a boolean ("true", "false") or "maybe"
+type triStateValue int
+
+const (
+	triStateFalse triStateValue = 0
+	triStateTrue  triStateValue = 1
+	triStateMaybe triStateValue = 2
+)
+
+const strTriStateMaybe = "maybe"
+
+func (v *triStateValue) IsBoolFlag() bool {
+	return true
+}
+
+func (v *triStateValue) Get() interface{} {
+	return triStateValue(*v)
+}
+
+func (v *triStateValue) Set(s string) error {
+	if s == strTriStateMaybe {
+		*v = triStateMaybe
+		return nil
+	}
+	boolVal, err := strconv.ParseBool(s)
+	if boolVal {
+		*v = triStateTrue
+	} else {
+		*v = triStateFalse
+	}
+	return err
+}
+
+func (v *triStateValue) String() string {
+	if *v == triStateMaybe {
+		return strTriStateMaybe
+	}
+	return fmt.Sprintf("%v", bool(*v == triStateTrue))
+}
+
+// The type of the flag as requred by the pflag.Value interface
+func (v *triStateValue) Type() string {
+	return "version"
+}
+
+func setUpFlagSet(tristate *triStateValue) *FlagSet {
+	f := NewFlagSet("test", ContinueOnError)
+	*tristate = triStateFalse
+	f.VarP(tristate, "tristate", "t", "tristate value (true, maybe or false)")
+	return f
+}
+
+func TestExplicitTrue(t *testing.T) {
+	var tristate triStateValue
+	f := setUpFlagSet(&tristate)
+	err := f.Parse([]string{"--tristate=true"})
+	if err != nil {
+		t.Fatal("expected no error; got", err)
+	}
+	if tristate != triStateTrue {
+		t.Fatal("expected", triStateTrue, "(triStateTrue) but got", tristate, "instead")
+	}
+}
+
+func TestImplicitTrue(t *testing.T) {
+	var tristate triStateValue
+	f := setUpFlagSet(&tristate)
+	err := f.Parse([]string{"--tristate"})
+	if err != nil {
+		t.Fatal("expected no error; got", err)
+	}
+	if tristate != triStateTrue {
+		t.Fatal("expected", triStateTrue, "(triStateTrue) but got", tristate, "instead")
+	}
+}
+
+func TestShortFlag(t *testing.T) {
+	var tristate triStateValue
+	f := setUpFlagSet(&tristate)
+	err := f.Parse([]string{"-t"})
+	if err != nil {
+		t.Fatal("expected no error; got", err)
+	}
+	if tristate != triStateTrue {
+		t.Fatal("expected", triStateTrue, "(triStateTrue) but got", tristate, "instead")
+	}
+}
+
+func TestShortFlagExtraArgument(t *testing.T) {
+	var tristate triStateValue
+	f := setUpFlagSet(&tristate)
+	// The"maybe"turns into an arg, since short boolean options will only do true/false
+	err := f.Parse([]string{"-t", "maybe"})
+	if err != nil {
+		t.Fatal("expected no error; got", err)
+	}
+	if tristate != triStateTrue {
+		t.Fatal("expected", triStateTrue, "(triStateTrue) but got", tristate, "instead")
+	}
+	args := f.Args()
+	if len(args) != 1 || args[0] != "maybe" {
+		t.Fatal("expected an extra 'maybe' argument to stick around")
+	}
+}
+
+func TestExplicitMaybe(t *testing.T) {
+	var tristate triStateValue
+	f := setUpFlagSet(&tristate)
+	err := f.Parse([]string{"--tristate=maybe"})
+	if err != nil {
+		t.Fatal("expected no error; got", err)
+	}
+	if tristate != triStateMaybe {
+		t.Fatal("expected", triStateMaybe, "(triStateMaybe) but got", tristate, "instead")
+	}
+}
+
+func TestExplicitFalse(t *testing.T) {
+	var tristate triStateValue
+	f := setUpFlagSet(&tristate)
+	err := f.Parse([]string{"--tristate=false"})
+	if err != nil {
+		t.Fatal("expected no error; got", err)
+	}
+	if tristate != triStateFalse {
+		t.Fatal("expected", triStateFalse, "(triStateFalse) but got", tristate, "instead")
+	}
+}
+
+func TestImplicitFalse(t *testing.T) {
+	var tristate triStateValue
+	f := setUpFlagSet(&tristate)
+	err := f.Parse([]string{})
+	if err != nil {
+		t.Fatal("expected no error; got", err)
+	}
+	if tristate != triStateFalse {
+		t.Fatal("expected", triStateFalse, "(triStateFalse) but got", tristate, "instead")
+	}
+}
+
+func TestInvalidValue(t *testing.T) {
+	var tristate triStateValue
+	f := setUpFlagSet(&tristate)
+	err := f.Parse([]string{"--tristate=invalid"})
+	if err == nil {
+		t.Fatal("expected an error but did not get any, tristate has value", tristate)
+	}
+}
diff --git a/example_test.go b/example_test.go
index 03ebeaa..6aaed3c 100644
--- a/example_test.go
+++ b/example_test.go
@@ -29,6 +29,10 @@
 	return fmt.Sprint(*i)
 }
 
+func (i *interval) Type() string {
+	return "interval"
+}
+
 // Set is the method to set the flag value, part of the flag.Value interface.
 // Set's argument is a string to be parsed to set the flag.
 // It's a comma-separated list, so we split it.
diff --git a/flag.go b/flag.go
index 47761a0..2e4cac0 100644
--- a/flag.go
+++ b/flag.go
@@ -466,7 +466,7 @@
 		return
 	}
 	if len(split) == 1 {
-		if _, ok := flag.Value.(*boolValue); !ok {
+		if bv, ok := flag.Value.(boolFlag); !ok || !bv.IsBoolFlag() {
 			err = f.failf("flag needs an argument: %s", s)
 			return
 		}
@@ -500,7 +500,7 @@
 			}
 		}
 		if alreadythere {
-			if _, ok := flag.Value.(*boolValue); ok {
+			if bv, ok := flag.Value.(boolFlag); ok && bv.IsBoolFlag() {
 				f.setFlag(flag, "true", s)
 				continue
 			}
diff --git a/flag_test.go b/flag_test.go
index 4d95b15..47865bd 100644
--- a/flag_test.go
+++ b/flag_test.go
@@ -245,6 +245,10 @@
 	return nil
 }
 
+func (f *flagVar) Type() string {
+	return "flagVar"
+}
+
 func TestUserDefined(t *testing.T) {
 	var flags FlagSet
 	flags.Init("test", ContinueOnError)