Ostia is one of the most extensively excavated cities in the Roman world and we are still analysing the ruins today. The ruins were excavated on a large-scale up to 1941, but were not documented in a scientific way. This has meant, in particuar, that the processes involved in the transition of Ostia to a late antique city (from the 3rd to the 5th c.) are still largely unknown. The idea put forward by Ostia’s most famous excavator, Guido Calza, that Ostia’s ‘end’ and irreversible decay began in the late 3rd c., had still influenced scholars until recently. The author’s research projects from 2001–2006 show that this is only true for certain areas of Ostia. Thanks to city-wide surveys and key-hole cleaning-excavations, the abandonment of predominantly commercial quarters can be verified from the 3rd c. onwards by the existence of blocked streets, ruins and rubbish dumps in these parts of the city. Yet, at the same time, we see the concentrated rebuilding of Ostia’s secular infrastructure in the 4th and early 5th c., including all major public buildings along the Decumanus and the creation of new ‘pedestrian-zones’, with the maintenance of both continuing into the later 5th c. at least.