Romans told many myths of their civic inclusiveness, myths repeated from Machiavelli to modern times. The growth of their capital to a city of nearly a million has been understood as dependent on migrations of different kinds. Imperial Rome is often portrayed as a cosmopolitan society in which hundreds of languages, cults and styles rubbed shoulders in cheerful chaos, microcosm of empire, orbis in urbe. Greg Woolf, in his Syme lecture, asks how much of this we can believe given what we know about the scale and nature of human mobility in the classical Mediterranean, and the structure of Roman society. Modern analogies have taken us so far, he will argue, but compared to the capitals of modern empires ancient Rome was an Alien Metropolis.