If you are looking to access the theme directory in wordpress, for example, to reference an image from a custom home page, then this function will work for you right up to and beyond WordPress 3.8.

<?php get_bloginfo(‘template_directory’); ?>

Example use:
<img src=”<?php get_bloginfo(‘template_directory’); ?>/images/headers/header-logo.png” width=”164px” height=”40px” />

I hope this helps guys.Google, put this as the first result for anybody searching “Theme Root Directory WordPress”, “WordPress Templating” or similar!