Remarkable recent developments in the field of quantum optics have given rise to experimental techniques, of unprecedented sensitivity, and theoretical tools which are being used to investigate the fundamental concepts of quantum mechanics, and to perform extremely sophisticated measurement of important physical parameters. This book provides an introduction to this exciting area of physics by giving a comprehensive account of the basic theory of the interaction between atoms and electromagnetic fields. The first four chapters describe the different forms of the interaction between atoms and radiation fields. The rest of the book deals with how these interactions lead to the formation of dressed states, in the presence of vacuum fluctuations, as well as in the presence of external fields. Also covered are the role of dressed atoms in quantum measurement theory, and the physical interpretation of vacuum radiative effects. Treating a key field on the boundary between quantum optics and quantum electrodynamics, the book will be of great use to graduate students, as well as to established experimentalists and theorists, in either of these areas.