This concise textbook introduces a graduate student to the various fields of physics related to the interaction between radiation and matter. The scope of the book is very broad, ranging from nonlinear to quantum optics and from quantum transitions in atoms to the dispersion of polaritons in continuous media.The author, Professor David Klyshko (1929–2000), is one of the founders of modern quantum optics, renowned for his theory of Spontaneous Parametric Down-Conversion (SPDC) and its applications in quantum metrology and the optics of nonclassical light.Most parts of the book contain the lecture courses taught by David Klyshko at Moscow State University, namely, quantum electronics, nonlinear optics and quantum optics. In every section, the main focus is on observable effects and their physical interpretation. The book emphasizes analogies and relations between seemingly different phenomena and different fields of quantum electronics. Additional commentaries written by Profs. Maria Chekhova and Sergey Kulik analyze more recent developments in the corresponding fields of physics.