AOL Instant Messenger, or "AIM", is a free internet messenger service sponsored and developed by AOL and is available for use on many operating system's, including Mac OS X, Microsoft Windows and Linux. It is a very popular Instant Messaging client and has one of the biggest user bases of all the instant messaging clients.

AOL Instant Messenger is a fully feature instant messaging client. It features status messages, video conferencing and a host of other unique features. Since the beginning, it has supported person-to-person chat, but most recently they have begun development on new features to allow larger conversations. In 2003, iChat was released which was the very first instant messaging client that was able to use the AIM protocol.

AOL Instant Messenger has a lot of competition in the instant messaging area. Competitors include Yahoo! Messenger , ICQ, Google Talk, Windows Live Messenger and The Trillian Astra Network. Almost all of these instant messaging clients support similar features as AIM.

AIM was originally developed and distributed in 1997-1998 upon the buy-out of ICQ. Each of these clients was treated as separate and never gained interoperability with each other.