The temple of Opet at Karnak, the mythical birthplace of Osiris at Thebes, though well known from an epigraphical point of view, is still poorly documented as far as its architecture is concerned. A systematic survey of the building has recently allowed a better understanding of the main features of its architectural programme: the existence of caissons filled with reused blocks carefully disposed by layers; the use of reused blocks of various periods and natures, including a stela of Tanutamun and an altar of Amenemhat III; thresholds made of granite or limestone which consist of almost systematically reused blocks; a cruciform plan; centring on divine figures. All these characteristics show the correlation established between the architectural programme and the theological programme of the temple.