Blood coagulation forms part of an integrated series of haemostatic reactions, involving plasma, platelet, and vascular components. Platelets adhere to damaged endothelium or to subendothelium under the influence of adhesive proteins, and when activated they aggregate and expose binding sites for coagulation factors. Platelets, therefore, act as vehicles to concentrate and potentiate coagulation reactions on the damaged vessels. Following interaction of the ‘contact’ factors XII and XI, the coagulation protease zymogens undergo sequential activation, resulting in the generation of thrombin, the conversion of fibrinogen to fibrin, and the formation of a platelet-fibrin haemostatic plug. The fibrinolytic system interacts to regulate fibrin deposition and removal during healing. Central to coagulation is the generation of thrombin. It is involved in promoting haemostatic reactions as well as a number of protective functions. The activities of thrombin and other serine proteases are modulated by the serine potease inhibitors (serpins), including antithrombin III and heparin cofactor II which are important in regulating the physiological anticoagulant action of glycosaminoglycans at the endothelium and the pharmacological action of heparin. Reduction of the formation or function of thrombin and other serine proteases is one of the primary aims of anticoagulant therapy.