The origin of this text was a request by industry and government to summarize the biological effects and to estimate the limits of safe exposure to longwave ul­ traviolet radiation. The specific issue was the safety of a small medium-pressure mercury arc designed to emit UV-A (NUVA-Lite, L. D. Caulk Co. , Milford, Delaware) for photopolymerization of resinous fillings used in dentistry. How-­ ever, the context grew to become a consideration of the risks and benefits to hu­ mans of electromagnetic radiation between the biologically active short UV and the visible spectrum. We have accumulated data from our own experimental work and from the literature and have attempted to put this information in the perspective of known biologic effects of ultraviolet radiation as it influences hu­ mans. Interest in the biological effects of longwave ultraviolet radiation is increas­ ing in all of the many scientific disciplines that make up the complex field of photobiology. In order to minimize the chance for error and personal prejudice and to maximize the use of expertise, each chapter has been reviewed by several authorities. Some of the contributions of this group led to significant alterations and creative additions to the chapter, and these persons deserve not only our sin­ cere gratitude but also recognition by the reader. These include Chapters 2 and 3: Dr. Robert E. Levin, Mr. Charles P. Comeau, Mr. Donald Gonser, Dr. David Sliney; Chapter 5: Dr. Jerry Williams, Dr. Robert Webb, Dr. Madhu A.