This is an overview of research developments in quantum optics over the past 20 years. In the early chapters, the formalism of quantum optics is elucidated and the main techniques introduced. These are applied in the later chapters to problems such as squeezed states of light, resonance fluorescence, laser theory, quantum theory of four-wave mixing, quantum non-demolition measurements, Bell's inequalities, and atom optics. Experimental results are used to illustrate the theory throughout.