The concepts and operational characteristics of cw laser injection locking to achieve compact, efficient, frequency stable CO2 transmitters are discussed. The basic physical phenomena involved and the role of gain saturation in laser injection locking are explored analytically by means of appropriate lower frequency and microwave circuits. Experimental results with a variety of regenerative and injection-locked CO2 amplifiers are presented and compared with the analytical predictions. To demonstrate the potential of the injection-locking technique, a 60 W sealed-off CO2 laser was locked in frequency to a 0.5 W stable oscillator. The experimental data are in close agreement with theory. A master-oscillator injection-locked transmitter for frequency stable operation was designed and tested. This experimental prototype design which was built to evaluate various concepts to be employed in a ruggedized frequency stable transmitter is described. A CO2 design suitable for frequency stable operation at 50 W, life tests with various metal-ceramic master-oscillator and amplifier discharge tubes and air-cooled operation are also discussed.