This book is an introduction to the two closely related subjects of quantum optics and quantum information. Essentially, the physical aspects of quantum information processing have now become an integral part of quantum optics. The book gives a simple, self-contained introduction to both subjects, while illustrating the physical principles of quantum information processing using quantum optical systems. It thus has an interdisciplinary character. For the benefit of a wider audience and to make the subject matter of the book accessible to those with backgrounds other than physics, the authors also include a brief review of quantum mechanics. Although much of the material used here can also be found in other books, discussed at various depths, the particular combination of the topics covered in this book is unique. Furthermore, some aspects of quantum information, for example those pertaining to recent experiments on cavity QED and quantum dots, are described here for the first time in book form.