SummaryPlatelet aggregation plays an important role in the pathogenesis of vascular thrombosis, although the stimulus to platelet aggregation differs in arterial and venous thrombosis. In arteries, platelets aggregate following the interaction between platelets and exposed subendothelial structures, in a ruptured atherosclerotic plaque. In veins, aggregation follows the activation of the intrinsic mechanism of coagulation and the generation of thrombin in areas of retarded blood flow. In both cases, the platelet aggregate is rapidly stabilised by fibrin.