This book is based on an introductory course of quantum electronics taught at Moscow Power Engineering Institute, National Research University, Russia. The book has seven chapters presenting the physics of modern quantum electronics devices such as lasers, masers and photodetectors. At the end of each chapter are problems, as in any textbook. The first chapter explains the properties of atoms and molecules using quantum mechanics. The next two chapters are dedicated to electric and magnetic dipole interactions. The rest of the book focuses on the field interactions with charges and matter, quantum amplifiers and generators, and the physics of lasers and masers. The book has a crystal-clear style and can be understood by any student with a basic physics and mathematical background.