Phototherapy exemplifies scientific medicine. The major advances have resulted from effective collaborations between basic researchers and clinicians. This book is directed to clinicians and basic researchers who are interested in current and emerging implementations of phototherapy. It can serve as an introductory reference and a textbook for advanced undergraduate and graduate courses in medical physics and biomedical engineering. The emphasis is on the science underlying the various phototherapy procedures, which encompasses aspects of classical and molecular photophysics, biological phohemistry, photobiology and biophotonics. Topics that do not usually appear in other general sources include the theory and applications of tissue optics, Monte Carlo simulation, light dosimetry, and analytical modeling of laser surgery. Many illustrative problems with answers are provided to exemplify the more quantitative aspects of each topic.