Pulmonary embolism involves blood clots in the lung (pulmonary emboli). Blood clots that end up in the lungs usually break off from clots in other parts of the body, such as the legs. Pulmonary emboli can block blood flow and lower oxygen levels. They can cause shortness of breath, coughing up of blood, and death. Doctors treat patients with pulmonary emboli with blood thinners. Blood thinners dissolve clots and prevent clots in the leg from breaking off and causing more pulmonary emboli. Because blood thinners can cause serious bleeding, it is important to use them only when necessary. There are many ways to diagnose pulmonary emboli, including special blood tests, different types of lung scans, radiographic and ultrasound tests to look for clots in the legs, and dye tests of the arteries in the lung (angiography). Some tests are more accurate than others, and none are perfect. Sometimes, doctors use many tests rather than single tests. Regardless, a main aim is to identify the patients who have no or very low likelihood of clots and do not need blood thinners. What does past research tell us about testing strategies that can be used to safely identify such patients?