The use of radiation in medicine provides great benefit to society by allowing for the diagnosis and treatment of different diseases and conditions. However, there are risks associated with using radiation, and practitioners must be knowledgeable in the principles of radiation protection and in the applications of radiation in medicine. Radiation protection describes a system of practices undertaken to minimize biological damage from ionizing radiation by both preventing acute effects and minimizing the likelihood of stochastic effects. The goal of radiation protection programs in medical settings is to minimize exposure to employees, patients, and the general public without reducing the quality of treatment. Different types of radiation and their applications generate different radiation protection challenges. Regardless of the radiation type or application, measurements of the radiation field and personnel dose (internal and external) are necessary. While measurements in medicine are performed the same way as in other settings, there are some differences across the different medical disciplines of diagnostic imaging (radiology), nuclear medicine, and radiation oncology. Each discipline employs different practices and technology for radiation field and personnel monitoring to keep radiation exposures as low as reasonably achievable (ALARA). These disciplines are treated separately in more detail in this chapter.