In this Memoir, a review is presented of the present knowledge on the geology of the Rupelian Boom Clay, in particular its sedimentation history and diagenetic evolution. A synthesis of the following items is given: the present occurrence of the deposit, the paleogeographical context of its formation, the stratigraphical position and subdivisions with their correlation potential, the lithology and the sedimentation model, the characteristic occurrence of septaria, the paleoclimatic conditions, the tectonic influences on the sedimentation and the post-Rupelian evolution of the clay mass.