Platelet-mediated primary haemostasis and blood coagulation have evolved as important defence mechanisms against bleeding. The formation of the platelet plug provides the initial occlusion of the vascular lesion. This is temporally co-ordinated with the activation of the coagulation system, which occurs in response to the rupture of endothelium and the exposure of blood to the extravascular tissue. The reactions of blood coagulation are carefully controlled by several anticoagulant mechanisms and under normal conditions they prevail over the procoagulant forces. Genetic or acquired disturbances of the natural balance between the pro- and anticoagulant systems may result in bleeding or thrombotic diseases.