The association of cancer with a hypercoagulable state is documented by numerous clinical, biochemical, pathologic, and pharmacologic studies. This association is manifested clinically by an increased incidence of intravascular thrombotic events in cancer patients and by fibrin deposition in and around tumor beds. Thromboembolic disease is a major cause of morbidity and mortality in patients with malignancy. This article discusses the complex pathogenesis of this problem and the associated laboratory and clinical syndromes with recommendations on diagnosis and treatment.