In one form or another psoralens have been in use dating back to biblical times for the treatment of depigmented patches of skin. However, it has only been in the past 40 years that the structure and function of psoralens have been elucidated. Although several volumes have been published on photobiology and photomedicine, no one volume has ever been devoted to the psoralen photobiology. In these two volumes we focus on the properties and uses of photoactivated psoralens.In these volumes the various aspects of psoralens are presented in a review of the field as it stands in mid 1986. In retrospect, we may find that this particular time was crucial in the development of new therapeutic modalities as many of the applications of modem molecular biology are beginning to impact on the practice of medicine.This book was written with two purposes in mind. First, to serve as an update (the last collective review of the field was in 1982). Second, it is hoped that newcomers to the fields of photobiology andphotomedicine both scientists and clinicians would find it a useful introduction.