This new and completely revised edition brings several new elements to the reader. Each chapter begins with aseries of Major Study Topics and con cludes with some Questions for Reviewand Discussion. I also have added a glossary to assist students with unfamiliar terms. This edition offers a greater emphasis on molecular biology and genetics than was present in either of the previous editions. The sequence of topics has also changed so that basic regulation and recombination are introduced early to provide a basis for subsequent discussion. I have preserved the pre sentation of basic virology and genetic transfer processes while expanding coverage of plasmid molecular biology. All of what I would consider to be essential material occurs within the first 13chapters. The final four chapters are shorter, optional material and are not interdependent; they can be used in any order or omitted at an instructor''s discretion. It is a pleasure to acknowledge the able assistance of the editorial and production staff of Springer-Verlag. I am also grateful to my colleagues who were so patient with my questions and to Rene Allard for his helpful com ments on the first six chapters.