diff --git a/README.md b/README.md
index d63f848..109c1d8 100644
--- a/README.md
+++ b/README.md
@@ -22,6 +22,8 @@ Create a new morgan logger middleware function using the given `format` and `opt
 The `format` argument may be a string of a predefined name (see below for the names),
 a string of a format string, or a function that will produce a log entry.
 
+For the syntax of strings and functions see [morgan.compile](#morgancompileformat).
+
 #### Options
 
 Morgan accepts these properties in the options object.
@@ -103,6 +105,14 @@ To define a token, simply invoke `morgan.token()` with the name and a callback f
 morgan.token('type', function (req, res) { return req.headers['content-type'] })
 ```
 
+Tokens can accept a string argument passed in from `[]` brackets by specifying
+a third argument `arg`:
+```js
+morgan.token('type', function (req, res, arg) { return arg })
+```
+
+Falsey values returned from this function will be replaced with `-`.
+
 Calling `morgan.token()` using the same name as an existing token will overwrite that token definition.
 
 ##### :date[format]
@@ -178,6 +188,11 @@ be passed using `[]`, for example: `:token-name[pretty]` would pass the string
 Normally formats are defined using `morgan.format(name, format)`, but for certain
 advanced uses, this compile function is directly available.
 
+The function returned takes arguments `tokens` , `req`, and `res` where `tokens`
+refers to `morgan` itself. If a log should be skipped the function will return
+`null`. The function returned or a custom function can be passed directly to 
+morgan using `morgan(myFn)`.
+
 ## Examples
 
 ### express/connect
@@ -310,6 +325,54 @@ function assignId (req, res, next) {
 }
 ```
 
+#### arguments to custom token formats
+
+Example of a custom token format that uses an argument. It will output the
+header related to the argument passed to the token.
+
+```js
+var express = require('express')
+var morgan = require('morgan')
+
+morgan.token('header', function getId (req, res, arg) {
+  return String(req.headers[arg])
+})
+
+var app = express()
+
+app.use(morgan(':header[host] :response-time'))
+
+app.get('/', function (req, res) {
+  res.send('hello, world!')
+})
+```
+
+### use custom format function
+
+Sample app that will use a custom formatting function. This will print JSON
+instead of a simple string.
+
+```js
+var express = require('express')
+var morgan = require('morgan')
+
+var formatter = morgan.compile('[:date[clf]] :method :url :res[content-length]')
+var JSONOutput = function (morgan, req, res) {
+  return JSON.stringify({
+    status: morgan.status(req,res),
+    default: formatter(morgan, req, res),
+  })
+}
+
+var app = express()
+
+app.use(morgan(JSONOutput))
+
+app.get('/', function (req, res) {
+  res.send('hello, world!')
+})
+```
+
 ## License
 
 [MIT](LICENSE)
diff --git a/test/morgan.js b/test/morgan.js
index 786cda1..7cc89ba 100644
--- a/test/morgan.js
+++ b/test/morgan.js
@@ -7,6 +7,7 @@ var join = require('path').join
 var morgan = require('..')
 var request = require('supertest')
 var split = require('split')
+var url = require('url')
 
 describe('morgan()', function () {
   describe('arguments', function () {
@@ -1316,6 +1317,102 @@ describe('morgan.compile(format)', function () {
   })
 })
 
+describe('morgan.token(name, function)', function () {
+  it('should overwrite existing functions if called multiple times', function (done) {
+    function token (req, res, arg) {
+      return arg && url.parse(req.url)[arg]
+    }
+    var urlToken = morgan.url
+    morgan.token('url', token)
+    assert.equal(morgan.url, token)
+    morgan.token('url', urlToken)
+    done()
+  })
+
+  describe('should use the string `-` if return value of the token function is falsey', function () {
+    function test (v, done) {
+      morgan.token('ret', function ret (req, res, arg) {
+        return arg
+      })
+      var cb = after(2, function (err, res, line) {
+        if (err) return done(err)
+        assert.equal(line, '-')
+        done()
+      })
+
+      var stream = createLineStream(function (line) {
+        cb(null, null, line)
+      })
+
+      var server = createServer(':ret', { stream: stream }, function (req, res, next) {
+        next()
+      })
+
+      request(server)
+      .get('/')
+      .expect(200, cb)
+    }
+    [
+      0,
+      NaN,
+      false,
+      '',
+      null,
+      undefined
+    ].forEach(function (v) {
+      it('for ' + (String(v) || JSON.stringify(v)), function (done) {
+        test(v, done)
+      })
+    })
+  })
+
+  it('should not see empty brackets as an argument value', function (done) {
+    morgan.token('checkempty', function ret (req, res, arg) {
+      assert.equal(arg, undefined)
+      assert.equal(arguments.length, 2)
+      cb(null)
+    })
+    var cb = after(3, function (err, res, line) {
+      if (err) return done(err)
+      assert.equal(line, '-[]')
+      done()
+    })
+
+    var stream = createLineStream(function (line) {
+      cb(null, null, line)
+    })
+
+    var server = createServer(':checkempty[]', { stream: stream }, function (req, res, next) {
+      next()
+    })
+
+    request(server)
+    .get('/')
+    .expect(200, cb)
+  })
+
+  it('should use the result', function (done) {
+    morgan.token('urlpart', function ret (req, res, arg) {
+      return url.parse(req.url)[arg]
+    })
+    var cb = after(2, function (err, res, line) {
+      if (err) return done(err)
+      assert.equal(line, 'foo=bar')
+      done()
+    })
+
+    var stream = createLineStream(function (line) {
+      cb(null, null, line)
+    })
+
+    var server = createServer(':urlpart[query]', { stream: stream }, function (req, res, next) {
+      next()
+    })
+
+    request(server).post('/test').query('foo=bar').expect(200, cb)
+  })
+})
+
 function after (count, callback) {
   var args = new Array(3)
   var i = 0