Although interest in coagulation has usually centered about the various hemorraghic disorders, it is hyperactivity of the hemostatic system with its thrombotic complications which has a far greater clinical impact. The hypercoagulability states are a vague group of disorders not well defined by the laboratory. They fall into two distinct groups: (1) conditions promoting venous thrombosis by activating the coagulation mechanism and (2) conditions promoting arterial thrombosis by platelet plug formation. Dealing with both of these groups separately the various disorders associated with a hypercoagulable state are discussed as well as their pathophysiologic basis; Special emphasis is placed on the laboratory evaluation of these disorders.