The presentation of PE is often subtle and may mimic other diseases. Many pulmonary emboli invariably preclude diagnosis by their occult nature or by leading to rapid death from cardiopulmonary arrest. In patients who do manifest symptoms from PE, accurate diagnosis is essential. Often it is difficult to distinguish the vague symptoms of PE from other diagnoses, such as acute coronary syndrome, pneumonia, COPD, CHF,aortic dissection, myocarditis or pericarditis, pneumothorax, and musculo-skeletal or gastrointestinal causes. Regardless of the presentation, the most fundamental step in making the diagnosis of PE is first to consider it. Historical clues and risk factors should raise the clinician's suspicion.PE is an unsuspected killer with a nebulous presentation and high mortality. In all likelihood, PE will remain an elusive diagnosis despite advances in technology and a wealth of research. A high index of suspicion is required, but no amount of suspicion would eliminate all missed cases. Patients with significant underlying cardiopulmonary disease seem to be the most challenging. Patients with significant comorbidity have poor reserve and are likely to have poor outcomes, especially if the diagnosis is not made and anticoagulation is not initiated early. Controversy exists over the best diagnostic approach to PE. A battery of diagnostic studies is available, with few providing definitive answers. Studies such as CT may be helpful at some institutions but offer poor predictive value at others. Other diagnostic tests are not universally available. It is hoped that further research and improvements in current diagnostic modalities will clear some of the current confusion and controversy of this ubiquitous and deadly disease.