Cavity quantum electrodynamics is about what happens to light and, in particular its interaction with matter when it is trapped inside a box. By light I mean not visible light (i.e., the optical region of the spectrum) but also the infrared and microwave regions. This book has two main aims. The first is to serve as an introductory text for Ph.D. students, postdocs, and more experienced physicists wishing enter this field. I have worked hard to write the book that I dreamed of reading when I started my Ph.D. The second aim is to serve as a supplementary text for an advanced undergraduate course in quantum mechanics.