This book provides a unified description of magnetic phenomena through means of the generalized susceptibility. The book begins by discussing the general properties of the susceptibility, its complex nature, its symmetry, and its relation to magnetic fluctuations.Next, the quantum basis of magnetic phenomena is established. The bulk of the book builds on this theoretical foundation to calculate the magnetic response to spatial and time-varying magnetic fields. This provides a natural way of thinking about long-range order, critical phenomena, resonance, etc. A new chapter is included which...