This volume attempts to fill the gap between the standard introductory textboks on solid state physics, and the specialized literature describing developments. It starts with the basic questions of the theory of magnetism, and goes on to include relatively elementary arguments to such topics of research interest as heavy fermions, Mott transition, and quantum magnetism. No specialized training in advanced theoretical methods is assumed. Though the text is primarily theoretical, it also discusses a number of experimental findings. The text is accompanied by problems, the solutions to which are worked out in detail.