This book discusses in a systematic manner the quantum properties of the light generated by various optical systems such as lasers and the effect of light on the dynamics of atoms. Several original and interesting procedures of analysis are presented. Some of these include the procedure followed to derive the master equation for a cavity mode or a two-level atom coupled to a thermal reservoir, to establish the correlation properties of the noise operators associated with the quantum Langevin equation, to obtain the input-output relation, and the approach adopted in the analysis of atom cooling. Moreover, local mean photon number, local quadrature squeezing, and the superposition of light beams are discussed. In addition, a refined quantum analysis of the light produced by subharmonic generators and lasers, pumped by electron bombardment or coherent light and coupled to a noiseless vacuum reservoir, is presented.