The clinical use of single photon emission computed tomography (SPECT) has grown steadily over the last decade. SPECT is now an essential technique for certain studies such as cerebral blood flow imaging. Many other common nuclear medicine studies give better results when they are performed with SPECT. These include myocardial perfusion imaging with thallium-201 or the new technetium-99m perfusion agents, myocardial infarct imaging with infarct-avid agents, imaging of tumors or infections with agents such as gallium-67 or indium-111 WBC's, and certain cases of bone imaging. Still other studies such as liver/spleen imaging, most brain studies, and perhaps renal imaging may benefit from SPECT even though planar imaging gives satisfactory results. Future developments in 3D display techniques and faster computers may extend the clinical usefulness of SPECT to other areas such as pulmonary perfusion imaging and gated cardiac blood pool imaging.