We know from an inscription that Trajan’s artificial harbor, whose completion marked the beginning of Ostia’s peak of prosperity, was built in A.D. 104. Ostia proper was at the very mouth of the Tiber, but silting, which today has put the beach of modern Ostia (Ostia Lido) three miles beyond the seawall of the ancient town, early made the city docks impracticable for any but the smallest vessels, so that Trajan built his harbor beside (indeed over the necropolis of) Claudius’, two-and-a-half miles northwest of the town. The traffic in grain and luxury goods to feed and pander to the more or less refined tastes of the largest and richest city in the world made Ostia vastly prosperous. The evidence is building activity, dated by brick stamps, impressed on building tiles, and bearing the names of consuls, tile manufacturers, or both. There was a slight time-lag, while prosperity built up. Only twelve per cent of the datable buildings in Ostia belong to Trajan’s reign; forty-three per cent were built or restored under Hadrian. Then activity tapers off again: seventeen per cent of the buildings are of Antonine date (A.D. 138–192), while only twelve per cent belong to the age of the Severi (A.D. 193–235). Thereafter Ostia, whose fortunes rose with Rome’s, declines with her also. The most illuminating way to describe what archaeology has to tell us about Ostia is to follow the plan used for Pompeii, treating in order the town and its population, municipal life and public amenities, housing arrangements, trade and industry, and the evidence for Ostia’s religious life. Art in Ostia hardly deserves separate treatment: it is, naturally, less well-preserved than at Pompeii, and what there is seldom rises above the level of pure documentation.