This book is based on a course of lectures for advanced students. Part 1 is devoted to an introductory treatment of general concepts and methods to be used for describing nonlinear processes. Part 2 is concerned with the application of these concepts and methods to significant effects and processes, covering also the particular experimental arrangements, measuring methods, and empirical data connected with them. In describing the methods and processes, the authors use as far as possible the classical or semiclassical theory. However, in the treatment of important problems concerning fluctuations, coherence and quantum processes of the field, the more complex fully quantum-theoretical description had to be employed. The authors have tried to render the book as self-sufficient as possible; in particular, Appendix A at the end is intended to provide a survey of the quantization of matter and of the electromagnetic field, with applicable formulas.