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_stylesheet_directory_uri(); using the correct one for the reasons stated.