diff --git a/README.md b/README.md
index 1f4ed1e..f39285c 100644
--- a/README.md
+++ b/README.md
@@ -18,7 +18,7 @@
 1. [Slices](#slices)
 1. [Maps](#maps)
 1. [Custom named decoders](#custom-named-decoders)
-1. [Custom decoders](#custom-decoders)
+1. [Custom decoders \(mappers\)](#custom-decoders-mappers)
 1. [Supported tags](#supported-tags)
 1. [Variable interpolation](#variable-interpolation)
 1. [Modifying Kong's behaviour](#modifying-kongs-behaviour)
@@ -162,11 +162,13 @@ This has the advantage that it is convenient, but the downside that if you modif
 A more robust approach is to break each command out into their own structs:
 
 1. Break leaf commands out into separate structs.
-2. Attach a `Run(...) error` method to all leaf commands (`Run()` signatures must match).
+2. Attach a `Run(...) error` method to all leaf commands.
 3. Call `kong.Kong.Parse()` to obtain a `kong.Context`.
-4. Call `kong.Context.Run(params...)` to call the selected parsed command.
+4. Call `kong.Context.Run(bindings...)` to call the selected parsed command.
 
-Note that `Run()` method arguments may also be provided by the `Bind(...)` option (see below).
+In addition to values bound with the `kong.Bind(...)` option, any values
+passed through to `kong.Context.Run(...)` are also bindable to the target's
+`Run()` arguments.
 
 There's a full example emulating part of the Docker CLI [here](https://github.com/alecthomas/kong/tree/master/_examples/docker).
 
@@ -362,7 +364,7 @@ specifies the element type. For maps, the tag has the format
 `tag:"[<key>]:[<value>]"` where either may be omitted.
 
 
-## Custom decoders
+## Custom decoders (mappers)
 
 If a field implements the [MapperValue](https://godoc.org/github.com/alecthomas/kong#MapperValue)
 interface it will be used to decode arguments into the field.
diff --git a/_examples/docker/main.go b/_examples/docker/main.go
index e678bf4..0fd01f3 100644
--- a/_examples/docker/main.go
+++ b/_examples/docker/main.go
@@ -2,24 +2,36 @@
 package main
 
 import (
+	"fmt"
+
 	"github.com/alecthomas/kong"
 )
 
 type Globals struct {
-	Config    string   `help:"Location of client config files" default:"~/.docker" type:"path"`
-	Debug     bool     `short:"D" help:"Enable debug mode"`
-	Host      []string `short:"H" help:"Daemon socket(s) to connect to"`
-	LogLevel  string   `short:"l" help:"Set the logging level (debug|info|warn|error|fatal)" default:"info"`
-	TLS       bool     `help:"Use TLS; implied by --tls-verify"`
-	TLSCACert string   `name:"tls-ca-cert" help:"Trust certs signed only by this CA" default:"~/.docker/ca.pem" type:"path"`
-	TLSCert   string   `help:"Path to TLS certificate file" default:"~/.docker/cert.pem" type:"path"`
-	TLSKey    string   `help:"Path to TLS key file" default:"~/.docker/key.pem" type:"path"`
-	TLSVerify bool     `help:"Use TLS and verify the remote"`
+	Config    string      `help:"Location of client config files" default:"~/.docker" type:"path"`
+	Debug     bool        `short:"D" help:"Enable debug mode"`
+	Host      []string    `short:"H" help:"Daemon socket(s) to connect to"`
+	LogLevel  string      `short:"l" help:"Set the logging level (debug|info|warn|error|fatal)" default:"info"`
+	TLS       bool        `help:"Use TLS; implied by --tls-verify"`
+	TLSCACert string      `name:"tls-ca-cert" help:"Trust certs signed only by this CA" default:"~/.docker/ca.pem" type:"path"`
+	TLSCert   string      `help:"Path to TLS certificate file" default:"~/.docker/cert.pem" type:"path"`
+	TLSKey    string      `help:"Path to TLS key file" default:"~/.docker/key.pem" type:"path"`
+	TLSVerify bool        `help:"Use TLS and verify the remote"`
+	Version   VersionFlag `name:"version" help:"Print version information and quit"`
+}
+
+type VersionFlag string
+
+func (v VersionFlag) Decode(ctx *kong.DecodeContext) error { return nil }
+func (v VersionFlag) IsBool() bool                         { return true }
+func (v VersionFlag) BeforeHook(app *kong.Kong, vars kong.Vars) error {
+	fmt.Println(vars["version"])
+	app.Exit(0)
+	return nil
 }
 
 type CLI struct {
 	Globals
-	VersionFlag bool `name:"version" help:"Print version information and quit"`
 
 	Attach  AttachCmd  `cmd help:"Attach local standard input, output, and error streams to a running container"`
 	Build   BuildCmd   `cmd help:"Build an image from a Dockerfile"`
@@ -65,7 +77,12 @@ type CLI struct {
 }
 
 func main() {
-	cli := CLI{}
+	cli := CLI{
+		Globals: Globals{
+			Version: VersionFlag("0.1.1"),
+		},
+	}
+
 	ctx := kong.Parse(&cli,
 		kong.Name("docker"),
 		kong.Description("A self-sufficient runtime for containers"),
@@ -73,11 +90,9 @@ func main() {
 		kong.ConfigureHelp(kong.HelpOptions{
 			Compact: true,
 		}),
-		//
-		kong.Hook(&cli.VersionFlag, func(ctx *kong.Context, path *kong.Path) error {
-			ctx.Printf("1.0.0").Exit(0)
-			return nil
-		}))
+		kong.Vars{
+			"version": "0.0.1",
+		})
 	err := ctx.Run(&cli.Globals)
 	ctx.FatalIfErrorf(err)
 }
diff --git a/context.go b/context.go
index 76e2c64..0a43daf 100644
--- a/context.go
+++ b/context.go
@@ -470,10 +470,10 @@ func (c *Context) parseFlag(flags []*Flag, match string) (err error) {
 	return findPotentialCandidates(match, candidates, "unknown flag %s", match)
 }
 
-// Run executes the corresponding Run(params...) method on the target command selected by the parsed args.
+// Run executes the Run() method on the selected command, which must exist.
 //
-// The target Run() method must exist and have the type signature "Run(params...) error".
-func (c *Context) Run(params ...interface{}) (err error) {
+// Any passed values will be bindable to arguments of the target Run() method.
+func (c *Context) Run(bindings ...interface{}) (err error) {
 	defer catch(&err)
 	node := c.Selected()
 	if node == nil {
@@ -487,7 +487,7 @@ func (c *Context) Run(params ...interface{}) (err error) {
 	if err != nil {
 		return err
 	}
-	binds := c.Kong.bindings.clone().add(params...).add(c)
+	binds := c.Kong.bindings.clone().add(bindings...).add(c)
 	return callMethod("Run", node.Target, method, binds)
 }
 
diff --git a/kong.go b/kong.go
index 8e3fdce..fb0bbf6 100644
--- a/kong.go
+++ b/kong.go
@@ -51,7 +51,7 @@ type Kong struct {
 	help          HelpPrinter
 	helpOptions   HelpOptions
 	helpFlag      *Flag
-	vars          map[string]string
+	vars          Vars
 
 	// Set temporarily by Options. These are applied after build().
 	postBuildOptions []Option
@@ -67,7 +67,7 @@ func New(grammar interface{}, options ...Option) (*Kong, error) {
 		Stderr:    os.Stderr,
 		registry:  NewRegistry().RegisterDefaults(),
 		resolvers: []ResolverFunc{Envars()},
-		vars:      map[string]string{},
+		vars:      Vars{},
 		bindings:  bindings{},
 	}
 
@@ -102,6 +102,8 @@ func New(grammar interface{}, options ...Option) (*Kong, error) {
 		return nil, err
 	}
 
+	k.bindings.add(k.vars)
+
 	return k, nil
 }