This book grew out of sets of lectures given to undergraduates taking courses in Biochemistry and Molecular Biology, and Medical Sciences. I hope it will be useful to people studying a range of biological subjects. I have tried to concentrate throughout on the general principles underlying the subject rather than to give overwhelmingly detailed accounts of vector systems and practical instructions. For those, there are more detailed books, reviews, catalogues, and lab manuals. I am grateful to the many friends and colleagues who have helped in the production of this book by reading sections (and in some cases the whole thing!) or in other ways. In particular, I should like to thank Janet Allen, Alison Baker, Adrian Barbrook, Alison Franklin, Hilary and Tony Larkum, and Saul Purton. I am also grateful to Robin Smith of Cambridge University Press for his advice and encouragement throughout the exercise, and to Robert Sugar and Dorothy Duncan of Bookworks for their help in the book's production.