In this book we attempt to summarize the present state of knowledge in the field of photoimmunology and to provide the background necessary to put this knowledge into perspective. This objective requires descriptive overviews in photobiology, immunology, and dermatology. Our hope is to facilitate interdis ciplinary communication and to stimulate interest in photoimmunology. Section I, Principles of Photoimmunology, describes (1) basic interactions between UV and visible radiation and biologic matter, (2) the optical properties of human skin, and (3) normal and abnormal responses to UV radiation. Basic principles of selected aspects of immunology and photobiology are explained. Section II, Experimental Photoimmunology, describes molecular, cellular, animal, and human studies in photoimmunology. Section III, Clinical Photo immunology, presents evidence of the involvement of photoimmunologic mechanisms in diseases of the skin and blood and examines the possible involve ment of immunologic mechanisms in therapeutic photomedicine. The funda mental principles outlined in this book should prove useful for decades to come. However, our expectation is that the experimental data will be outdated in a short period of time. The potential of photoimmunology is great enough and the serious study of photo immunology is recent enough that this should be the case. This book is dedicated to the goal of continuing rapid growth in the field of photoimmunology .