Books on quantum electronics and laser technology contain usually information only on ideal empty resonators. At the same time, this information is only a starting point in an analysis of the non-linear interaction of the resonator modes, evolution of these modes and so on, processes which are intimately connected with the active medium and affect in a decisive way the characteristics of the laser radiation. The principal purpose of the present book is filling this gap, primarily in what concerns the problems related with the spatial characteristics of laser radiation. It contains not only material bearing on the specific features in the behaviour of resonators filled with a gain medium, the main principles to be followed in selecting the types and parameters of resonators based on the given properties of a medium, but all the relevant information on the laws governing the propagation of coherent light beams and on empty ideal resonators. This book sums up the research conducted by the author for many years which was directed to a large extent toward systematization of theoretical and experimental data scattered through the literature, development of convenient terminology, a system of representation and unique methods to analyze resonators of different types.