The use of single photon emission computed tomography (SPECT) as the procedure for screening, assessing the size and number of embolized areas, and evaluating the follow-up of patients with pulmonary thromboembolism (PTE) is reviewed. Details of the technique for acquiring and processing perfusion and ventilation lung SPECTs are presented. The tomographic images produced by SPECT allow the application of a less-expensive diagnostic algorithm than that classically used. This is due to the fact that the SPECT images contain more anatomical and physiological information than similar planar images. The signs of PTE in perfusion SPECT and the significance of mismatch, match, and paradoxical mismatch are discussed and illustrated with examples.