The diagnosis of venous thromboembolism, which comprises deep vein thrombosis and pulmonary embolism, remains challenging. Combining pretest probability of disease with the results of noninvasive testing, such as ventilation-perfusion lung scan and duplex ultrasound, can avoid costly and invasive testing with pulmonary angiography and contrast venography in most patients. Techniques to estimate the pretest probability of deep vein thrombosis and pulmonary embolism are described.