Joomla Access Control Lists (ACL) is one of the most popular and powerful features in Joomla. ACL controls two things: 1) what users can do on a website and 2) what users can see on a website. Individual Users can be placed into User Groups which are assigned Permissions and Access Control Levels. The robust core ACL helps site administrators to create tiered levels off access to different areas of the site and it can support thousands of concurrent users. Naturally, because the ACL system is so powerful, it has attracted many users to Joomla, but it also raises a lot of questions.