This chapter describes some special radionuclides which have great potential in nuclear medicine. These radionuclides are divided into two groups: radionuclides that are clinically proven useful and radionuclides that have great promise for future clinical use. This radionuclide has been used primarily for bone marrow imaging, either by positron scintillation cameras or by dual probe rectilinear scanners. The major difficulty with this radionuclide is its high energy gamma radiations of 446 and 511 keV, which are not ideal for imaging with a scintillation camera. Thallium-201 is commercially available carrier-free in the chemical form of thallous chloride. In this monovalent form it behaves like alkali metal ions, and therefore lends itself as a useful radionuclide for various clinical applications including myocardial visualization, tumor uptake, and thyroid uptake. Imaging is performed at rest and during exercise by means of a scintillation camera.