Rome, the old capital of a world empire, lost its position as seat of imperial governance already in the 3rd century AD but by the end of antiquity had become the centre of an ecclesiastical empire. The city was transformed into a new entity now dominated by the church. In this changing urban matrix a distinctive Christian architectural practice developed. We shall analyse the principal architectural and artistic outcomes of this process from the material reorganization of the church in the time of Constantine until the age of ecclesiastical reform. We will also be considering the lived fabric of the city, on which much new light has been thrown in recent years.