There is a recurrent need for high power, frequency-stable lasers in applications as diverse as laser radar, remote sensing, gravitational wave interferometry, and nonlinear optics. This need is often satisfied by using a low power, frequency-stable laser followed by a chain of amplifiers, but a preferred approach is to injection-lock a high power (slave) laser to a lower power, frequency-stable (master) laser. Other advantages of the injection-locking technique are the possibility of ensuring single-mode operation, eliminating mode partition noise, mode hopping, preventing spurious feedback effects, and synchronizing one or more free-running lasers to the same pump. As explained in Section 3.4, the main benefits of optical injection occur when the frequencies of both lasers are close together and for sufficiently large injected power. The slave laser then gets the spectral properties of the master one in terms of frequency and linewidth. Stover and Steier did the first optical injection experiment in 1966 using gas lasers. The first optical injection experiment using semiconductor lasers (SLs) came much later and was done by Kobayashi and Kimura in 1980. At that time, it was not clear that SLs would be useful lasers, but the performance of these lasers has dramatically improved during the last 30 years, providing reliable devices for a large variety of applications. Optical injection is used to reduce noise (frequency noise, mode partition noise, or intensity noise, to generate microwave signals, or to produce chaotic outputs for secure communication.