The Proceedings presented here contain the notes of lectures delivered during the Eleventh Winter School of Theoretical Physics, held at Karpacz, Poland, February 19 - March 4, 1974. The School was primarily devoted to new concepts in the theory of magnetism in metals, alloys, and metallic compounds, but, as can be seen from the table of contents of the book, other topics of the theory of magnetism were also discussed in the course of the lec- tures. The organizers agreed to such a broadening of the scope in order to satisfy particular requests from the Polish participants for whose benefit...