I think you have to be a little careful because it depends on what you are trying to do. 

If you are using a child theme         get_template_directory(); will still go to the parent theme.  However get_stylesheet_directory(); will go to the current theme, child or parent. Also, both these functions return absolute server paths. 

If you wanted a fully formed URI, for links or images, you should use get_template_directory_uri(); or get_stylesheet_directory_uri(); using the correct one for the reasons stated. 

### Summary ###

* [`get_stylesheet_directory()`][1]: file path to current Theme directory
* [`get_stylesheet_directory_uri()`][2]: url path to current Theme directory
* [`get_template_directory()`][3]: file path to parent Theme directory
* [`get_template_directory_uri()`][4]: url path to parent Theme directory


  [1]: http://codex.wordpress.org/Function_Reference/get_stylesheet_directory
  [2]: http://codex.wordpress.org/Function_Reference/get_stylesheet_directory_uri
  [3]: http://codex.wordpress.org/Function_Reference/get_template_directory
  [4]: http://codex.wordpress.org/Function_Reference/get_template_directory_uri