This book represents the proceedings of the NATO Advanced Study Institute held in Santa Flavia, Sicily from the 20 - 29th June, 1977. In addition to the review talks given by the Lecturers at the Institute it proved feasible for other topics to be splendidly reviewed. This has led to a much wider subject coverage than would otherwise have been possible. The discussion sessions which followed these review talks were extremely valuable and almost all the participants played an active role. Essentially all of the verbal contributions presented at this ASI were subsequently put into written format, which is why these proceedings are so extensive. ~hey do, however, provide an up-to-date summary of DNA synthesis in a wide variety of subjects with many of the remaining problems clearly expressed. The editing of these contributions has been essentially confined to alterations in style and presentation. We have taken some liberties in the re-organization of the papers into related sections. We express our thanks to those who helped organize the ASI and to the session conveners who attempted to confine and contain those who became too verbose. We are indebted to NATO, Scientific Affairs Division for the financial support that made this ASI possible. Finally, we express our gratitude to Miss Brenda Marriott. She typed all seventy five papers in this book, which was originally estimated to be less than half its present length and which just grew and grew. She deserves our special thanks.