Blood must fulfill two purposes that can be reconciled only with difficulty: It should remain fluid in order to assure the nutrition of the organs; and it must, in case of injuries, solidify rapidly in order to avoid a dangerous blood loss. Blood coagulation with its inhibitors and antagonists serves this double purpose. It is completed by additional mechanisms of hemostasis, that is, vascular and platelet reactions. The present report will discuss only the most extensively studied disorders of blood coagulation. Clinicians are often accused of having complicated excessively the simple coagulation scheme proposed by physiologists in the