This book is intended for the student who is taking a first course in bacterial and bacteriophage genetics, rather than as a reference tool for the specialist. It presumes a knowledge of basic biology as weIl as familiarity with general genetics. Extensive knowledge of microbiology, although helpful, is not essen tial for a good understanding of the material presented herein. In order to develop the basic concepts of bacterial and bacteriophage genetics in a volume of reasonable size, I have endeavored to avoid the stricdy molecular approach as weIl as the thoroughly comprehensive treatment characteristic of review articles. For simplification and continuity, therefore, I have dealt primarily with Escherichia coli and its phages, except where other bacteria can better illustrate a particular point. This should not, however, be construed to imply that only E. coli is worthy of study. Rather , it is my hope that students will be able to generalize from the principles presented in this book to the specific bacterial systems which may be of more direct interest to them.