When the first edition of this book was published in 1950, it set out to present an elementary outline of the state of knowledge of nucleic acid biochemistry at that time and it was the first monograph on the subject to appear since Levene's book on Nucleic Acids in 1931. The fact that a tenth edition is required after thirty five years and that virtually nothing of the original book has been retained is some measure of the speed with which knowledge has advanced in this field. As a result of this vast increase in information it becomes increasingly difficult to fulfil the aims of providing an introduction to nucleic acid biochemistry and satisfying the requirements of advanced undergraduates and postgraduates in biochemistry, genetics and molecular biology. We have attempted to achieve these aims by con­ centrating on those basic aspects not normally covered in the general biochemistry textbooks and by providing copious references so that details of methodology can readily be retrieved by those requiring further information. The first seven editions emerged from the pen of J. N. Davidson who died in September 1972 shortly after completing the seventh edition. The subsequent editions have been produced by various colleagues who have tried to retain something of the character and structure of the earlier editions while at the same time introducing new ideas and concepts and eliminating some of the more out -dated material.