This book originated in numerous Gordon Research Conferences and many other meetings of scientists working in chemistry, biophysics, biochemistry, and biology related to mutagenesis and carcinogenesis. It seemed the appro­ priate time to sit back and summarize the results of several decades of research in laboratories in different countries. We are very grateful to the Rockefeller Foundation for inviting us to formulate and begin writing the book at the Center for International Studies in Bellagio, Italy, where we were Resident Scholars. We are fortunate to have had the assistance of so many colleagues around the world who cheerfully sent original data, figures, and preprints and lis­ tened patiently to us as we worked out the various conflicting ideas in this fast-moving. field. The names of these scientists are found within the tables, figures, and references. There is one person whose contributions we especially wish to acknowl­ edge. Professor Heinz Fraenkel-Conrat was present at the inception of this book and throughout the writing encouraged and criticized in apprOximately equal proportion. Finally, his editing and amalgamation of our two styles gave us great comfort. B.S. D.G.