Cancer is a frequent finding in patients with thrombosis, and thrombosis is much more prevalent in patients with cancer, with important clinical consequences. Thrombosis is the second most common cause of death in cancer patients. Venous thromboembolism (VTE) in cancer is also associated with a high rate of recurrence, bleeding, a requirement for long-term anticoagulation, and worsened quality of life. Risk factors for cancer-associated VTE include particular cancer types, chemotherapy (with or without antiangiogenic agents), the use of erythropoietin-stimulating agents, the presence of central venous catheters, and surgery. Novel risk factors include platelet and leukocyte counts and tissue factor. A risk model for identifying cancer patients at highest risk for VTE has recently been developed. Anticoagulant therapy is safe and efficacious for prophylaxis and treatment of VTE in patients with cancer. Available anticoagulants include warfarin, heparin, and low-molecular weight heparins (LMWHs). LMWHs represent the preferred therapeutic option for VTE prophylaxis and treatment. Their use may be associated with improved survival in cancer, although this issue requires further study. Despite the significant burden imposed by VTE and the availability of effective anticoagulant therapies, many oncology patients do not receive appropriate VTE prophylaxis as recommended by practice guidelines. Improved adherence to guidelines could substantially reduce morbidity, decrease resource use, enhance quality of life, and improve survival in these patients.