The field of quantum optics has witnessed significant developments in recent years, from the laboratory realization of counter-intuitive concepts such as lasing without inversion and micromasers, to the investigation of fundamental issues in quantum mechanics, such as complementarity and hidden variables. This book provides an in-depth and wide-ranging introduction to the subject of quantum optics, emphasizing throughout the basic principles and their applications. The book begins by developing the basic tools of quantum optics, and goes on to show the application of these tools in a variety of quantum optical systems, including resonance fluorescence, lasers, micromasers, squeezed states, and atom optics. The final four chapters are devoted to a discussion of quantum optical tests of the foundations of quantum mechanics, and particular aspects of measurement theory.