Congenital causes of venous thrombosis have gained increasing prominence with the description of the factor V Leiden mutation and the prothrombin gene mutation. More recently, the description of the association between increased levels of coagulation factors and venous thrombosis and the finding that patients with thrombophilia can harbor more than one prothrombotic state have further increased the clinical relevance of the congenital thrombophilic states. In this qualitative review, we summarize current knowledge of the congenital prothrombotic states and propose a simple classification system that divides the states into two broad groups: those associated with reduced levels of the inhibitors of the coagulation cascade and those associated with increased levels or function of the coagulation factors. The first group is less common than the second, but it is associated with a much higher risk for venous thrombosis. This review provides clinicians with an evidence-based, practical guide to the congenital prothrombotic states.