This book is formulated from a number of presentations made at a one-day workshop on the subject of Photon Migration in Tissues. The meeting was held in Philadelphia at the University of Pennsylvania, April, 1988. The workshop was an impromptu effort to bring together scientists to discuss photon migration in animal tissues and appropriate models. The rapid emergence of the ideas of Townes and Schalow in their invention of the then called maser, now laser opened up completely unexpected possibilities for biomedical research. Timing of rapid biochemical reaction, identification of unstable intermediates, spectroscopy of short lived fluorescent states were all goals to be expected and achieved. At the same time continuous light spectroscopy of tissue slices and of the myocardium, and eventually of the brain have the to the the neonate emerged over years. Shifting red end of spectrum, Butler and Norris clearly showed how transparent plant materials and the human hand could be illuminated in this region and Jobsis applied their idea to the neonate brain using a multiwavelength technique.