The concept of using photochemical probes in the study of biological systems was developed by Westheimer who published the first photoaffinity labeling experiments more than twenty years ago (J.Bio1. Chem. 237, (1962) 3006). Since then the concept has been used successfully in various areas of biochemistry and recently several new interesting and exciting aspects of the concept have been developed. It is the general opinion by scientists in the field that the full potential of photochemical probes in biochemical studies has far from been exploited yet. This is mostly due to the interdisciplinary character of the concept involving photochemistry, synthetic chemistry as well as biochemistry/molecular biology. The perspective of the NATO advanced workshop on Photochemical Probes in Biochemistry , held in Holte (Copenhagen) Denmark 14-19, August, 1988, was several fold. The workshop was to give an account of the state of the art of using photochemical probes in biochemistry as well as to bring together specialists in photochemistry, syn thetic chemistry and molecular biology in order to analyze advantages as well as the inherent problems and pitfalls of the concept and provide suggestions and guidelines for fu ture research. Furthermore, it is the hope of the editor that the present publication which gives an account of the lectures presented at the workshop, will provide an introduction to scientists who are not familiar with photochemical probes, but to whom these could help answer central and pertinent questions.