Radioactive tracers have been in use in medical diagnostics for a long time and even today, the thyroid gland continues to be most frequently examined organ in nuclear medicine. Over 10,000 hospitals worldwide use radioisotopes in medicine, and about 90 per cent of the procedures are diagnostic. Radioisotope is characterized by the type and energy of radiation emitted and also by its characteristic life-time. Production, handling, detection and uses of radioisotopes have been included in this chapter. Radioisotopes used in biological experiments are known as tracers. Tracers are generally short-lived isotopes linked to chemical compounds which permit specific physiological processes to be scrutinized. They do not upset the natural biological processes. They are used in determining the steps and paths by which molecular reactions occur, in plotting the time and space distribution of biologically active chemicals and in plotting the course of fluid flow. Many counters are employed for detection and measurement of radiations emitted by radioactive material. A brief outline of ionizing chamber, proportional, Geiger-Muller and scintillation counters have been provided to complete the coverage of subject matter. External exploration of the organ for knowing distribution of radioactive material within organ is done by using moving detector imaging system and the process is called scintography. Use of radionuclides for obtaining scintogram of thyroid, pancreas, liver, lung, brain, kidney and heart have been outlined. Keeping in view usefulness of radioisotopes in autoradiography, radioimmunoassay, radiodating, and sterilization of foods and equipment, this topic is included in text.