The concept of citizenship in the ancient world — who was a citizen, what rights and obligations citizenship entailed, how it could be acquired or lost — reveals fundamental assumptions about political community that differ from modern concepts in instructive ways.

Athenian citizenship was ethnic and genealogical: citizenship required descent from citizen parents (after Pericles' law of 451 BC, from two Athenian citizen parents). It was not available for purchase, not easily granted to foreigners, and not transferable. The exclusivity of Athenian citizenship meant that the large resident foreigner population (metics) — often wealthy, educated, and contributing to Athenian cultural life — had no political standing regardless of how long they had lived in Athens.

Roman citizenship was different in fundamental ways: it was extensible, could be granted to individuals and communities, and was eventually extended to virtually all free inhabitants of the empire (the Constitutio Antoniniana of 212 AD). The process by which Italian allies gained citizenship after the Social War of 91–87 BC, followed by the gradual extension through the provinces, shows Roman citizenship as a political technology for integrating conquered peoples.

The grant of citizenship to freed enslaved people (libertini) in Rome was another distinctive feature: a freed slave became a Roman citizen of inferior status, and his children were full citizens. This mechanism of social mobility — available in principle to many thousands of enslaved people in Roman society — had no parallel in the Greek world and created a significant pathway between the conditions of slavery and full civic participation.