LASERLASER is an abbreviation of Light Amplification by Stimulated Emission of Radiation. The research on maser and laser was named “quantum electronics”, expressing the concept that these are new electronic amplifiers in the microwave and optical frequency regions which are enabled by the quantum–mechanical concept of stimulated emission of radiation. Although we can describe laser light as a superposition of coherent electromagnetic waves which are formalized by the electromagnetic theory, the quantized photons play essential roles, for example, in the spontaneous emission which starts the laser action, creating entangled photon pairs for quantum teleportation, and reducing the quantum noise of the detectors. In this chapter, the concepts of photons and quantum mechanics are briefly introduced, followed by quantization of the optical field. Then absorption, spontaneous emission, and stimulated emission of photons is described. Finally, the gain coefficient for light amplification is derived as the basic parameter in laser operation.