Hypercoagulable states can be inherited or acquired. Inherited hypercoagulable states can be caused by a loss of function of natural anticoagulant pathways or a gain of function in procoagulant pathways. Acquired hypercoagulable risk factors include a prior history of thrombosis, obesity, pregnancy, cancer and its treatment, antiphospholipid antibody syndrome, heparin-induced thrombocytopenia, and myeloproliferative disorders. Inherited hypercoagulable states combine with acquired risk factors to establish the intrinsic risk of venous thromboembolism for each individual. Venous thromboembolism occurs when the risk exceeds a critical threshold. Often a triggering factor, such as surgery, pregnancy, or estrogen therapy, is required to increase the risk above this critical threshold.