The last thirty years have witnessed a profound increase in our understanding of the ways in wh ich organic compounds react together-their mechanisms of reaction. This has, on the one hand, become a large, discrete branch of organic chemistry; but it has also, on the other, had a considerable impact on our approach to devising methods for the synthesis of organic compounds. To the student, reaction mechanism can have a two-fold appeal: it is, in its own right, an intellectually stimulating subject in its rationalization and unification of complex processes; and it also provides a relatively simple superstructure on which the vast array of the facts of organic chemistry can be hung. In a para- doxical way, the amount to be usefully learned in a subject to which an array of facts is being added daily remains, as our understanding grows, alm ost unchanged. The purpose of this book is to show how an understanding of these mechanis- tic principles can usefully be applied in thinking ab out and planning the con- struction of organic compounds. It is designed for those who have had abrief introduction to organic chemistry; an elementary knowledge of the nomencla- ture and structures of organic compounds is assumed. The text is divided into two parts.