For libraries that specify autoload information, Composer generates a vendor/autoload.php file. You can simply include this file and you will get autoloading for free.

require __DIR__ . '/vendor/autoload.php';

This makes it really easy to use third party code. For example: If your project depends on Monolog, you can just start using classes from it, and they will be autoloaded.

$log = new Monolog\Logger('name');
$log->pushHandler(new Monolog\Handler\StreamHandler('app.log', Monolog\Logger::WARNING));
$log->addWarning('Foo');

You can even add your own code to the autoloader by adding an autoload field to composer.json

{
    "autoload": {
        "psr-4": {"Acme\\": "src/"}
    }
}

Composer will register a PSR-4 autoloader for the Acme namespace.

You define a mapping from namespaces to directories. The src directory would be in your project root, on the same level as vendor directory is. An example filename would be src/Foo.php containing an Acme\Foo class.

After adding the autoload field, you have to re-run dump-autoload to re-generate the vendor/autoload.php file.

Including that file will also return the autoloader instance, so you can store the return value of the include call in a variable and add more namespaces. This can be useful for autoloading classes in a test suite, for example.

$loader = require __DIR__ . '/vendor/autoload.php';
$loader->add('Acme\\Test\\', __DIR__);

In addition to PSR-4 autoloading, Composer also supports PSR-0, classmap and files autoloading.