Although it is one of the oldest physical phenomena studied, magnetism con tinues to be an active and challenging subject. This is due to the fact that mag netic phenomena represent a complex application of quantum mechanics, statistical physics, and electromagnetism. As new magnetic materials are syn thesized and new experimental conditions realized, the very fundamentals of these subjects are expanded. Thus, the Kondo effect, like superconductivity, stimulated the development of many-body techniques; spin glasses with their competing interactions are leading to advances in statistical physics; and angle and spin-resolved photoemission is probing details of transition-metal electronic states never before possible. I have not tried to incorporate all the new developments in this subject since the first edition ten years ago. My purpose is still the same - to use linear response theory to establish a common conceptual basis for understanding a variety of magnetic phenomena. Many recent developments fit into this frame work and have been included.
