Optical imaging and spectroscopy use light emitted into opaque media such as human tissue to determine interior structure and chemical content, respectively, and have broad application to the field of medicine . Few developments have improved medical diagnos- tics as much as the ability to noninvasively peer inside the body, and it is expected that newly developing optical imaging techniques will continue this trend. Optical imaging and spectroscopy, key components of optical tomography, center around the simple idea that light passes through the body in small amounts, emerging bearing clues about tissues through which it passed. Rapid progress over the past decade, made possible by the collective output of multiple laboratories and advancements in the opto-electronics field, have brought optical imaging to the brink of clinical usefulness. In recognition of this impending transition from lab curiosity to medical tool, it was felt that a comprehensive review was in order. This book, written in large part by members of the key laboratories responsible for recent optical advances, aims to fill that need by exploring state-of-the-art methods, hardware, and applications of optical tomography. It is intended to serve as both an introduction for those unacquainted with the field, as well as a reference for those actively involved. It is hoped that the reader will gain an appreciation of the power and breadth of optical tomography, as well as an understanding of the fundamental limitations and unsolved problems that need study before optical imaging becomes clinically viable. Lastly, we hope to communicate the excitement and enthusiasm felt by those studying optical imaging and spectroscopy, and demonstrate why the field is undergoing both an explosion of interest and rapid progress.