Although venous thrombosis has been traditionally associated with stasis and hypercoagulability, arterial thrombosis is mainly associated with heightened platelet reactivity and damage to the vessel wall. Accordingly, classic risk factors for venous and arterial thrombosis are usually considered distinct. Those for the former include cancer, surgery, pregnancy, and estrogens use, whereas risk factors of arterial thrombosis include smoking, hypertension, diabetes, the metabolic syndrome, and hyperlipidemia. However, a number of studies have recently challenged this dichotomy, and it is now recognized that venous and arterial thromboses share several risk factors, suggesting a closer link between the two clinical conditions. Typical examples of shared risk factors are age and the metabolic syndrome. This review addresses the mechanism whereby established risk factors increase the risk of venous or arterial thrombosis, or both.