This book presents a phenomenological approach to the field of solid state magnetism. Beginning with basic concepts in statistical thermodynamics and electronic structure theory, the text discusses 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. The book will be a useful reference for researchers and a valuable accompaniment to graduate courses on magnetism and magnetic materials.