The sensors of light or other electromagnetic radiations are referred to as photodetectors. A photodetector has a p-n junction that converts the light photons into current. They can be broadly classified on the basis of their mechanism of detection into photoemission, thermal polarization and photochemical. Photoemission photodetectors use photoelectric effect where electrons transition from conduction bands of a material to free electron state. The most common examples of photodetectors are photodiodes and photo transistors. Some of the fundamental properties of photodectors are quantum efficiency, detectivity, dark current, responsivity, noise-equivalent power, nonlinearity and noise spectrum. Photodetectors include various devices such as gaseous ionization detectors, photomultiplier tubes, microchannnel plate detectors, active pixel sensors, charge-coupled devices, photoresistors, cryogenic detectors, etc. This book traces the progress of this field and highlights some of its key concepts. It is compiled in such a manner, that it will provide an in-depth knowledge about the technology and applications of this field. Scientists and students actively engaged in this field will find this book full of crucial and unexplored aspects of this domain.