Coloniae (sing. colonia) were settlements or colonies established by the state to form a self-administering community, often with a strategic defensive function. Most colonies were founded on state-owned land, but sometimes they were established on land belonging to a municipium — an existing town incorporated into the Roman state, whose inhabitants might or might not be Roman citizens.

Colonies were founded from an early stage in Rome's history, with three Italian colonies apparently established during the monarchy and 11 more before 338 BCE. These early colonies were settled by Romans to protect Rome from hostile Italian tribes. After the defeat of the Latin League in 338 BCE, the Latin colonies (composed largely of Romans who did not have Roman citizenship) were independent states that were controlled by Rome only in matters of foreign affairs. They also had obligations to supply contingents to the Roman army. By the end of the 2nd century BCE, colonies were established at key points throughout Italy.

As Rome's territory expanded in the late republic and early empire, increasing numbers of colonies were founded outside Italy as a means of establishing loyal communities in newly captured areas. Most settlers were usually retired legionaries, and in the 1st century BCE, the main purpose of founding colonies was to provide land for legionary veterans. Within Italy, land was purchased or confiscated, and confiscation caused much discontent. Julius Caesar founded a large number of colonies, mainly outside Italy. Colonization appears to have reached a peak under Augustus, although subsequent emperors, such as Claudius, were eager to found colonies in the newest provinces.

The title colonia was also conferred on some existing provincial towns. After Hadrian's reign this became common, and the founding of colonies in unsettled areas became rare. Even small settlements (such as those attached to fortresses and forts) could be upgraded as they developed, eventually being given the status of colonia.