For some time now there has been an interest in the nop. ­ linear interaction of electromagnetic waves in plasma [1,2]. But only in the last few years has the theory of nonlinear wave inter­ action effects undergone such vigorous development as to result in the formulation of clear phYSical concepts regarding the mech­ anisms of interaction. This development has been engendered by attempts to solve many of the plasma-physical problems accom­ panying the tremendous growth of experimental research [3]. The importance of nonlinear effects in modern plasma physics is dis­ cussed in detail in Chap. I. At this point we merely stress the fact that today the analysis of nonlinear effects is a practical ne­ cessity in any experiment involving plasma instabilities. We should also point out that plasma instabilities can assert them­ selves extensively in solids (solid state plasma) and play an im­ portant part in the study of cosmic plasma. Consequently, the problems of nonlinear wave interaction in plasma are of concern to those working in widely different areas of physics. Yet it is difficult to assimilate the results of investigations on nonlinear effects, owing to the complicated way in which the results of orig­ inal research are presented. In the present book the author hopes in some measure to fill the need for a text on the physics of non­ linear effects that is accessible to a fairly general audience.