This study deals with hitherto unpublished evidence of private religious practice, found in the ruins of Ostia. The selected environment consists of domus, rented apartments, workshops, depots, shops, bars, markets, hotels, and also all mithraea and the compita. These chapters contain concise introductions to the categories of buildings, including lists, notes on the dates, appearance, lay-out, size and distribution, and notes on the owners, inhabitants, visitors, workers and personnel.