It is widely held as uncontroversial that throughout the classical period male inhabitants of Attica were divided between three distinct categories – Athenian citizens, metics (regularly translated as ‘resident aliens’) and slaves – and that Athenian society had, therefore, a tripartite structure. The opportunities available to and requirements demanded of a man depended on his category. Those foreigners permanently resident in Attica – those with the legal status of ‘metic’ – were, unlike slaves, free, but, unlike citizens, they could not own land, vote in the Assembly, or serve as a dikastes or as a magistrate; in addition, metics were required to pay a poll tax (the metoikion) and to have a citizen sponsor (prostates). In this paper I seek to challenge not the nature of the distinction between citizens and metics but instead the assumption that the distinction was made throughout the classical period. I suggest that the growth of the Athenian citizen population after the Persian invasion of 480–479 demands that the origin of metic status be situated around the middle of the fifth century, and that the occasion on which the Athenian polis first defined metic status is likely to have been the occasion on which it first took an interest in restricting who might become a citizen: in 451/0, with the passing of Perikles' citizenship law.