This book is a graduate-level text covering the fundamental physics and chemistry required for a modern understanding of the interstellar medium. Radiation mechanisms are comprehensively presented, and extensive examples are drawn from observations in the X-ray, ultraviolet, optical, infrared, mm/sub mm, and radio observations. This book goes beyond a phenomenological study of the interstellar medium to give a detailed quantitative treatment of the radiative and dynamical interactions between stars and the interstellar medium. With an emphasis on a physical understanding of these processes, the mathematical derivations are clean, elegant and easily understandable by anyone with an undergraduate background in physics.