Platelets, derived from megakaryocyte cytoplasm, have a critical role in normal haemostasis, and in thrombotic disorders. The development of megakaryocytes is controlled by thrombopoietin, which binds to c-mpl on the surface of platelets and megakaryocytes. Platelet membrane glycoproteins mediate binding to subendothelial tissue and aggregation into haemostatic plugs. Thrombocytopenia and disorders of platelet function cause petechiae and mucocutaneous bleeding. Drugs causing specific inhibition of platelet function are important in the treatment of cardiovascular and cerebrovascular disease.