The basic principles of laser injection locking are discussed and the status of recent experimental work is reviewed. The subject is introduced by means of appropriate lower frequency and microwave injection-locking circuits. These are used to explore the basic physical phenomena involved and to illustrate the essential role of gain saturation in laser injection locking. Oscillators and below-threshold regenerative amplifiers are discussed to provide a more complete and cohesive picture. Experimental results with a variety of CW and quasi-CW injection-locked amplifiers including unstable resonator geometries are presented. Preliminary results with a novel injection-locked passively Q-switched CO2amplifier are also given.