AIM (alternative investment market) is a market run by the London Stock Exchange for trading in companies too small to list on its main market.

The requirements for an AIM listing are much less stringent, and it is far cheaper to list on AIM than on the main market. AIM is designed for smaller companies. Despite having over 2,000 companies listed, it is much smaller than the main market in terms of total market cap.

AIM does benefit from sharing trading systems and other infrastructure with the main market. Successful AIM companies often go on to list on the main market.