Loading Backbone and Underscore using RequireJS

Update: As of version 1.3.0 Underscore removed AMD (RequireJS) support.

You can use the amdjs/Backbone 0.9.1 and the amdjs/Underscore 1.3.1 fork with AMD support from James Burke (the maintainer of RequireJS).

More info about AMD support for Underscore and Backbone.

// main.js using RequireJS 1.0.7
require.config({
    paths: {
        'jquery': 'libs/jquery/1.7.1/jquery',
        'underscore': 'libs/underscore/1.3.1-amdjs/underscore', // AMD support
        'backbone': 'libs/backbone/0.9.1-amdjs/backbone', // AMD support
        'templates': '../templates'
    }
});

require([
    'domReady', // optional, using RequireJS domReady plugin
    'app'
], function(domReady, app){
    domReady(function () {
        app.initialize();
    });
});

The modules are properly registered and there is no need for the order plugin:

// app.js
define([
    'jquery', 
    'underscore',
    'backbone'
], function($, _, Backbone){
    return {
        initialize: function(){
            // you can use $, _ or Backbone here
        }
    };
});

Underscore is actually optional, because Backbone now gets its dependencies on its own:

// app.js
define(['jquery', 'backbone'], function($, Backbone){
    return {
        initialize: function(){
            // you can use $ and Backbone here with
            // dependencies loaded i.e. Underscore
        }
    };
});

With some AMD sugar you could also write it like this:

define(function(require) {
    var Backbone = require('backbone'),
        $ = require('jquery');

    return {
        initialize: function(){
            // you can use $ and Backbone here with
            // dependencies loaded i.e. Underscore
        }
    };
});

Regarding the optimizer error: doublecheck your build configuration. I assume your path configuration is off. If you have a directory setup similar to the RequireJS Docs you can use:

// app.build.js
({
    appDir: "../",
    baseUrl: "js",
    dir: "../../ui-build",
    paths: {
        'jquery': 'libs/jquery/1.7.1/jquery',
        'underscore': 'libs/underscore/1.3.1-amdjs/underscore',
        'backbone': 'libs/backbone/0.9.1-amdjs/backbone',
        'templates': '../templates'
    }, 
    modules: [
        {
            name: "main"
        }
    ]
})

For reference, as of version 1.1.1 (~Feb '13), Backbone also registers itself as an AMD module. It will work with requirejs without the need to use its shim config. (James Burke's amdjs fork also hasn't been updated since 1.1.0)

RequireJS 2.X now organically addresses non-AMD modules such as Backbone & Underscore much better, using the new shim configuration.

The shim configuration is simple to use: (1) one states the dependencies (deps), if any, (which may be from the paths configuration, or may be valid paths themselves). (2) (optionally) specify the global variable name from the file you're shimming, which should be exported to your module functions that require it. (If you don't specify the exports, then you'll need to just use the global, as nothing will get passed into your require/define functions.)

Here is a simple example usage of shim to load Backbone. It also adds an export for underscore, even though it doesn't have any dependencies.

require.config({
  shim: {
    underscore: {
      exports: '_'
    },
    backbone: {
      deps: ["underscore", "jquery"],
      exports: "Backbone"
    }
  }
});

//the "main" function to bootstrap your code
require(['jquery', 'underscore', 'backbone'], function ($, _, Backbone) {   // or, you could use these deps in a separate module using define

});

^{Note: this simplified code assumes that jquery, backbone and underscore are in files named "jquery.js", "backbone.js" and "underscore.js" in the same directory as this "main" code (which becomes the baseURL for require). If this isn't the case, you'll need to use a paths config.}

I personally think with the built-in shim functionality, the advantages of not using a forked version of Backbone & Underscore outweigh the benefits of using the AMD fork recommended in the other popular answer, but either way works.