this book is mainly about the fundamental ideas underlying the field of cavity quantum electrodynamics. it adopts a model-building approach and covers a number of topics that are not covered by any other quantum optics textbook (cavity qed can be considered a subset of the broader field of quantum optics). certain topics that have been included are: 1. a detailed discussion of the idea of the photon ` s wavefunction and photon mass 2. an introduction to the problem of the vacuum catastrophe and the cosmological constant. 3. apedagogical account of the theory of multiplication of distributions 4. an introduction to quaternions. special attention has been paid to the historical background of the topic.