Radiation detectors are divided according to their working principle. Solid-state detectors are semiconductors and based on the concept of creating electron–hole pairs. There are many types of solid-state detectors, but this chapter concentrates on the most commonly used diodes made from silicon and diamond, as well as MOSFETs. Physical properties, working principles, applications, and limitations are discussed. The solid-state detectors can be used in other branches of medicine, but this chapter deals mainly with radiation therapy practice, including external irradiation and brachytherapy, photon, electron, and charged particle beams.