In 1999, two blocks of shops were pulled down in the centre of the city of Mainz on a site that in Roman times was not far from the major road that led from the camp of leg. XIV Gemina to the bridge over the Rhine. This chapter provides basic information about the sanctuary of Isis and Mater Magna. It comprises several cult-rooms and – as usual in mystery-cults – rooms for meetings and banquets, also a well and a latrine. The sanctuary is of very considerable importance for the social and religious history of Germania Superior, and indeed the north-western provinces of the Roman Empire. The joint sanctuary of Isis and Mater Magna at Mainz and the lead tablets found there are important in this context for three reasons.