Writing a textbook on microbial genetics in about 200 pages was un doubtedly a difficult task, but I have been encouraged by the response from both students and lecturers to the first edition. The requirement for a second edition is also a measure of the need for such a book. My experience as a lecturer has shown that what is needed first is an intelligible framework which can be read in a reasonable period of time. Armed with these principles, a student can then go to reviews and the original literature with a reasonable chance of understanding the jargon and the details. Molecular genetics is now so well advanced that it is easy to lose track of the purpose of a set of experiments in the wealth of sequence data and complex interactions. I have therefore kept the same format for this edition with a well-illustrated text giving original papers, popular reviews, monographs and detailed reviews to enable the student to take the subject further as required.