Quantum Electronics, Volume 2: Maser Amplifiers and Oscillators deals with the experimental and theoretical aspects of maser amplifiers and oscillators which are based on the principles of quantum electronics. It shows how the concepts and equations used in quantum electronics follow from the basic principles of theoretical physics. Comprised of three chapters, this volume begins with a discussion on the elements of the theory of quantum oscillators and amplifiers working in the microwave region, along with the practical achievements in this field. Attention is paid to two-level paramagnetic masers and the dependence of the form of the emitted signal on the different parameters. The maser oscillator operating with a beam of active molecules is described in the next chapter, which considers the three-level paramagnetic oscillator, the molecular beam oscillator, and the two-level solid-state quantum oscillator. The final chapter is devoted to lasers, including gas lasers and solid-state lasers. Methods of obtaining negative temperatures are described, together with the elements of laser theory and the kinetics of oscillation processes in solid-state lasers. This book is intended for university students with knowledge of theoretical physics, particularly quantum mechanics.