The natural history of pulmonary embolism (PE) is incompletely characterized, because most episodes of PE go undetected, the clinical presentation mimics so many other common and uncommon diseases, the sensitivity and specificity of the diagnostic tests are poorly defined, and even detection at autopsy is difficult and requires close examination of the pulmonary arteries. Yet PE is a significant cause of morbidity and mortality in the hospitalized patient, and one reason for its extremely high incidence is the failure of physicians to provide adequate prophylaxis to patients who are at risk of developing venous thromboembolism. The mortality rate for PE is less than 8% when the condition is recognized and treated correctly but approximately 30% when untreated. Pulmonary arteriography is still the gold standard in diagnosing pulmonary emboli, but several other imaging modalities have been used to diagnose pulmonary emboli in recent years, including transthoracic and transesophageal echocardiography, magnetic resonance angiography, spiral computerized tomography, and ventilation-perfusion lung scanning. The treatment modality chosen depends directly on the clinical presentation of the patient. Low molecular weight heparin may be equal or superior in efficacy to unfractionated heparin for the treatment of deep venous thrombosis and PE. Thrombolytic therapy can be considered for patients with hemodynamic instability, those with right ventricular dysfunction, and young patients with a massive PE despite a normal right ventricle on echocardiography. In those patients who cannot receive anticoagulation therapy or thrombolysis, or who remain at high risk, an inferior vena cava filter should be placed.