This book presents a phenomenological approach to the field of solid state magnetism. After introducing the basic concepts from statistical thermodynamics and electronic structure theory, the first part discusses the standard models for localized moments (Weiss, Heisenberg) and delocalized moments (Stoner). This is followed by a chapter about exchange and correlation in metals, again considering the results for the localized and delocalized limit. The book ends with a chapter about spin fluctuations, which are introduced as an alternative to the finite temperature Stoner theory. A useful reference work for researchers, this book will also be a valuable accompaniment to graduate courses on magnetism and magnetic materials.