SummaryThromboses are a major complication of several cardiovascular diseases. The following factors intervene in their formation. First, the haemodynamic factor can explain the differences in the genesis of venous and arterial thromboses. Second, the parietal factor depends on the subendothelial structures—collagen fibres, basal membranes, microfibrils—which can all fix the circulating platelets The latter are a third and essential factor in the genesis of thromboses. After their adhesion to the subendothelial structures, they undergo a mutual aggregation, leading to the formation of mural or occlusive thrombi.The understanding of the factors determining platelet adhesion and aggregation and the discovery of agents inhibiting these processes will allow an efficient treatment of incipient thromboses.