The decay of a radioactive nucleus leads to the emission of energy in the form of photons or charged particles. The form and energy of the radiation emitted will depend on the decaying nucleus. Some of the emitted energy will be absorbed by target organs; the ratio of the absorbed energy to the mass of the target is the radiation dose. Charged particles traveling in a medium slow down because of interactions between the electric charge of the particle and that of the orbital electrons and nuclei of the medium. These interactions transfer energy from the charged particle to the orbital electrons and the nuclei of the medium. A photon may be transmitted through a medium or may be attenuated by the medium. Of the four mechanisms by which photons interact with matter, two are important in the energy range of interest in nuclear medicine: the photoelectric effect and Compton scattering. The internal radiation dose from radionuclides used in nuclear medicine can be estimated with the Medical Internal Radiation Dose (MIRD) method. Important aspects of the MIRD method include the concepts of source and target organs, energy emitted per decay, absorbed fraction, S value, cumulated activity, and effective dose equivalent.