This book evolved from a course in molecular biology which I have been teaching primarily to graduate students for the past twenty years. Because the subject is now mature, it is possible to present the material by covering the principles and encouraging students to learn how to apply them. Such an approach is particularly efficient as the subject of molecular genetics now is far too advanced, large, and complex for much value to come from attempting to cover the material in an encyclopedia-like fashion or teaching the definitions of the relevant words in a dictionary-like approach. Only the core of molecular genetics can be covered by the present approach. Most of the remainder of the vast subject however, is a logical extension of the ideas and principles presented here. One consequence of the principles and analysis approach taken here is that the material is not easy. Thinking and learning to reason from the fundamentals require serious effort, but ultimately, are more efficient and more rewarding than mere memorization.