Pulmonary embolism (PE) is a common and potentially lethal disease; unfortunately, the diagnosis is often missed because patients with PE present with nonspecific signs and symptoms. If left untreated, approximately one third of patients who survive an initial PE subsequently die from a future embolic episode. Most patients succumb to PE within the first few hours of the event. In patients who survive, recurrent embolism and death can be prevented with prompt diagnosis and therapy. Virchow identified a triad of factors that lead to the pathogenesis of venous thrombosis: venous stasis, injury to the intima, and changes in the coagulation properties of the blood. Pulmonary emboli usually arise from the thrombi originating in the deep venous system of the lower extremities; however, rarely they may originate in the pelvic, renal, or upper extremity veins and the right heart chambers. After travelling to the lung, large thrombi lodge at the bifurcation of the main pulmonary artery or the lobar branches and cause hemodynamic compromise. Smaller thrombi continue travelling distally, occluding a smaller vessel in the lung periphery. These are more likely to produce pleuritic chest pain by initiating an inflammatory response adjacent to the parietal pleura. Most pulmonary emboli are multiple, and the lower lobes are involved more commonly than the upper lobes. Immediate full anticoagulation is mandatory for all patients suspected to have DVT or PE.