Writing this second edition of Biochemical Genetics proved to be more difficult than I had anticipated. The fixed format of the series meant that the addition of new material was made possible only by the dele tion of old. Since the book is intended for a student audience, I have retained the historical approach of the first edition and added new material only when it demonstrates a principle more effectively. At the time of writing, we are witnessing an information explosion resulting from the application of recombinant DNA technology to all manner of problems. I have added a sixth chapter indicating the impact of this work on our concepts of gene structure. I should like to thank Ed Byard, Bill Evans, Charles Schorn and Ed Ward, colleagues in the Biology Department at the University of Winnipeg, and Andrew Spence, a student in the department, for their comments on the manuscript of the second edition, and to reiterate my thanks to all those in the Department of Genetics at the University of Sheffield who commented on the first edition.