Nuclear medicine is a specialized area of radiology that uses very small amounts of radioactive materials, or radiopharmaceuticals inside the body to examine organ function and structure for diagnosis or to target and destroy damaged or diseased organs or tissue (for treatment). Several different types of radionuclides are available. These include forms of the element technetium, thallium, gallium, iodine, and xenon. Out of these radionuclides, thallium's radioisotope plays a crucial role in the diagnosis of chronic ailments like CAD, myocardial infarction, ischemia, to name few. These diseases cause specific muscles or arteries to die or get blocked. Out of the radioisotopes of thallium, Tl-201mis is considered best suited for diagnosis of such diseases by tomography techniques such as Single Photon Emission Computed Tomography (SPECT). In this article, we have presented an overview of how thallium can be used as a nuclear medicine to test the stress level in human, mechanism of action as a tracer and perfusion imaging test by using SPECT.