An ant colony is an underground lair where ants live. Colonies consist of a series of underground chambers, connected to each other and the surface of the earth by small tunnels. There are rooms for nurseries, food storage, and mating. The colony is built and maintained by legions of worker ants, who carry tiny bits of dirt in their mandibles and deposit them near the exit of the colony, forming an anthill.

Ant colonies are eusocial, and are very much like those found in other social Hymenopterans, though the various groups of these probably developed sociality independently through convergent evolution. Eggs are laid by one or sometimes more queens. Queens are different in structure, they are the largest ones among all ants, especially their abdomens and thorax which are bigger than most ants. Their tasks are to lay eggs and produce more offspring. Most of the eggs that are laid by the queens grow up to become wingless, sterile females called workers. Periodically swarms of new queens and males called alates are produced, usually winged, which leave to mate. The males die shortly thereafter, while the surviving queens either found new colonies or occasionally return to their old one. The surviving queens can live up to around 15 years.

People raise ant colonies in captivity for research and as a hobby. An "ant terrarium" used for this purpose is called a formicarium. They are often made thin enough that you can see the entire colony inside their nest. These are called ant farms.

Until 2002, the largest known ant colony was on the Ishikari coast of Hokkaido, Japan. The colony was estimated to comprise of 306 million worker ants and 1 million queen ants living in 45,000 interconnected nests over an area of 2.7 km². In 2002 a super-colony of connected nests was found to stretch nearly 6000 km across Europe, and another, measuring approximately 100 km wide, was found beneath Melbourne, Australia in 2004.