The principle of operations and the development and applications of silicon detectors are discussed. The application of strip detectors in high-energy physics follows Moore's law in both the area and the channel count. New developments include pixel and drift detectors. Examples of use in space sciences and the growing use in medical applications are given. The role of silicon detectors in the detection of photons is explained. Frequent reference will be made to papers submitted to the IEEE2000 NSS-MIC.Index Terms-Applications in astrophysics, applications in medical sciences, applications in particle physics, pixel detectors, silicon, silicon gamma detectors, silicon photon detectors, silicon strip detectors, tracking detectors.