The definition of radiation units and various radiation-related terms are given in this chapter. Sources of radiation exposure, and calculation of external radiation exposure and internal absorbed dose are presented in detail. Several pertinent tables related to absorbed doses are included in the chapter. A table includes absorbed doses to different organs from the administration of different radiopharmaceuticals to humans. The NRC dose limits to radiation workers, the public, minors, and pregnant women are given. Effective dose is defined and explained, and its values from different radiopharmaceuticals are presented in a table.