Radiation induces a variety of chemical processes in biological tissues. This volume is a synthesis of up-to-the-minute reviews on such phohemical and photobiological sensitized reactions with particular relevance to photomedicine. The first part gives a description of experimental techniques for the study of the primary processes after radiation absorption by biological systems. It is followed by chapters on singlet oxygen and photomedicine, considering both phototherapy and phohemotherapy. These sections also discuss the next generation of potential photosensitizing drugs.