This book presents the fundamental concepts of the theory, illustrated by numerous examples of astrophysical applications. Classical concepts are combined with new developments and the authors demarcate what is well established and what is still under debate. To book illustrates how apparently complicated phenomena can be addressed and understood using well-known physical principles and equations within appropriate approximations and simplifications. For this purpose, a number of astrophysical examples are considered in greater detail than what is normally presented in a regular textbook. In particular, a number of nonlinear self-consistent models are considered, which is motivated by the latest observational data and modern theory.