An accurate diagnosis of pulmonary embolism is essential to prevent excessive morbidity and mortality from lack of therapy or inappropriate anticoagulation. Clinical signs and symptoms are reported to be nonspecific, although published studies do not allow calculation of true specificity. Since certain clinical characteristics or groups of findings may be sensitive enough for pulmonary embolism, the diagnosis is unlikely in their absence. Ventilation-perfusion lung scanning has high sensitivity but variable specificity for pulmonary embolism. Patients with scans showing multiple segmental or lobar perfusion defects with normal ventilation have a high probability of pulmonary embolism. Scans with less extensive perfusion abnormalities or matching ventilation defects do not reliably exclude pulmonary embolism. Pulmonary angiography is the most definitive procedure for diagnosing pulmonary embolism. Digital subtraction pulmonary angiography and radiolabeled platelet scanning are promising but require more extensive validation before routine use.