Nuclear medicine is a medical specialty, where diagnostic and or therapeutic radioisotopes are used to study the physiology of organs and the metabolism of various types of tumors. Pharmaceuticals labeled with radionuclides (radiopharmaceuticals) are studied at pre-clinical level before being used in humans. Animals (Rodents) are generally used to study the biokinetics of tracer in a group of predefined organs. The extrapolation of the results of these studies from animals to humans provides an estimate of the behavior of the radiopharmaceuticals and the irradiation delivered clinically. Nuclear Medicine is fundamentally based on Radiopharmaceuticals whose biodistribution in disease and healthy organ result in either images that are diagnostically useful or local irradiation of tissue that is therapeutically beneficial for treatment of tumors. In result, in most procedures the biodistribution is primarily dependent on clearance of the radiopharmaceuticals from the blood into organs, tissues or lesions. Radiation is harmful for living beings and hence radiation toxicity is required to assess for new radiopharmaceutical which can be calculated by following the methodology of Internal dose calculation. Basic principle of Internal dosimetry and calculation methodology are explained in this chapter.