Photodetectors are widely employed as optical receivers to convert light into electrical signals in many applications ranging from communications to electronics, medicine, automotive, and transport. In gamma-ray astronomy, photodetectors are extensively used by space experiments to detect and measure the energy of X-rays and gamma rays (from a few keV to hundreds of GeV) interacting in various scintillator materials. Moreover, they are required in the cameras of Imaging Atmospheric Cherenkov Telescopes (IACTs) on the ground to measure very short light pulses produced by the interaction of high-energy gamma rays in the atmosphere down to a level of several photons. In this chapter, we introduce representative photodetectors used for high-energy astronomy, including photomultiplier tubes (PMTs), solid-state (semiconductor) photodetectors, with particular emphasis on silicon photomultipliers (SiPMs), and silicon drift detectors (SDDs). We address their basic properties and highlight past, present, and future applications, with particular emphasis to their employment in many small CubeSat missions devoted to the study of the high-energy, multimessenger sky.