Organic chemistry is one of the most rapidly developing of the sciences. Each year, there are new applications of organic compounds, for example, in medicine, in agriculture, and in the new technologies such as optoelectronics and supercon- ductors; tens of thousands of new compounds are synthesized or isolated from natural sources; and new synthetic methods and reagents are introduced. It might seem to the student that ever more needs to be learned. Fortunately that is not so. The profound increase in our understanding of the pathways by which organic compounds react - their mechanisms of reaction - has provided a relatively simple superstructure on which the vast array of the facts of the subject can be hung. The mechanistic principles are relatively few, and yet they account for the enormous range of reactions of organic compounds. The purpose of this book is to show how these principles can be applied both to acquiring a knowledge of organic synthetic processes and to planning the construction of organic compounds. It is designed for those who have had no more than a brief introduction to organic chemistry. Nor is it intended to be com- prehensive; for example, the vast body of evidence on which reaction mechanisms are based is not included, nor are experimental details given. The object has been to convey a broad understanding rather than to produce a reference text.