This book was written during a period when the technologies of genetic engineering were being applied to the study of animal viruses and when the organization and function of individual virus genes were being elucidated. This book, which uses human and animal viruses as models, aims to under stand the developments in molecular virology during the last 20 years. Al though molecular virology could also be taught by means of bacteriophages or plant viruses, the advantage of using animal viruses is in their ability to cause human and animal diseases as well as to transform cells, a primary problem in medicine. For the sake of clarity and convenience, not all the individual contributors to the various aspects of molecular virology were cited in the text. Instead, the reader is referred to review articles or key papers that list the numerous excel lent publications that have contributed to clarification of the various molecular processes. Thus the end-of-chapter bibliographies will guide the reader to the publications in which the original contributing authors are quoted. References given under the heading Recommended Reading are intended to assist those interested in pursuing a given subject further. I hope that this book will fulfill the purpose for which it is designed, and I urge readers to contact me if errors are found or updating is required.