The changes between the third and fourth editions are evolutionary in nature, both literally and figuratively. New in this edition are some boxed questions for students to read and think about before continuing on to the next section. The Thinking Ahead boxes are intended to prod the students to make predictions based on their existing knowledge before reading the new material. The Appli cations boxes are intended to encourage the students to try their hands at exper imental design. The answers to these latter questions are provided in an appen dix, usually in the form of a reference to a research paper that addressed that specific question. For instructors who have used the previous edition of this book, there is only one major rearrangement of material. All the discussion on generalized re combination occurs in Chapter 5. Chapter 15 is now devoted entirely to site specific recombination. Once again, the final four chapters are shorter and much vii Preface to the Fourth Edition less dependent on one another, so they can be used in any order or omitted at an instructor''s discretion. Previously, each new edition had a new set of genetic maps for the major bacteria and bacteriophages. This practice has now fallen vic tim to the success of the genetics community.