Summary Summary: Blood coagulation and fibrinolysis are mechanisms that, in parallel with inflammatory and repair responses, help to protect the integrity of the vascular system and to maintain the fluidity of blood. The initial response to vascular injury is contraction of vessels. The normal endothelium maintains blood fluidity by producing inhibitors of blood coagulation and platelet aggregation, by modulating vascular tone and permeability, and by providing a protective envelope separating haemostatic blood components from reactive subendothelial structures. Thus, loss or injury of endothelium results in a shift towards increased blood coagulation. Adhesion of platelets and platelet aggregation requires von Willebrand factor, and platelet glycoproteins as well as formation of thromboxan from arachidonic acid are subsequent steps which result in a platelet plug at the site of injury. Fibrin formation, through the activation of plasma coagulation occurs on this initial platelet plug and stabilises the thrombus. Several plasma procoagulant factors are involved in this reaction. Specific inhibitors regulate fibrin formation in order to limit the reaction and to prevent excessive fibrin formation. Loss of procoagulant factors results in a bleeding diathesis, loss of these inhibitors in thrombophilia. The fibrinolytic system is able to lyse clots or thromboseses. It is similarly organised by promoters and inhibitors. This system is also regulated by hormones and cytokines, which can induce or increase expression, synthesis and release of haemostasis factors. A sufficient function of haemostasis is essential in critical care patients, since bleeding and thrombosis are frequent and relevant complications in critical care patients. Laboratory evaluation of haemostasis focuses on platelet counts and plasma coagulation profiles. The detailed knowledge of the test systems is important since therapy with antithrombotic drugs, blood components or coagulation factor concentrates is frequent, expensive and not without risks in these patients.