Pericyclic reaction - the third type of organic reaction mechanism along with ionic and radical reactions - include some of the most powerful synthetically useful reactions, like the Diels - Alder reaction, 1,3- dipolar cycloadditions, the Alder ene reaction, Claisen rearrangements, the 2,3-Wittig rearrangement, diimide reduction, sulfoxide elimination and many others. These reactions are characterised by having cyclic transition structures, and also have highly predictable stereochemical features. Every organic chemist must be able to recognise the various types of pericyclic reaction and know something of their mechanisms and the factors that affect how well they work in organic synthesis. This book identifies the four main classes of pericyclic reaction, and discusses the main characteristics of the most important class, cycloadditions - providing a working knowledge, based on real examples, of their scope, patterns of reactivity, and stereochemistry. Then it explains the main features using ideas based in molecular orbital theory, but ( as in the companion book by A. J. Kirby on Stereoelectronic Effects ) without mathematics. It presents the Woodward - Hoffmann rules in the form of two all-encompassing rules, one for thermal reactions and its opposite for photochemical reactions. These rules are explained in detail and carefully illustrated, so that you will be able to predict the stereochemical outcome for any pericyclic reaction. The remaining chapters use this theoretical framework to show how the rules work with the other three classes of pericyclic reactions - electrocyclic reactions, sigmatropic rearrangements and group transfer reactions. By the end of the book, you will be able to recognise any pericyclic reaction and predict with confidence whether it is allowed, and with what stereochemistry, and you will have a working knowledge of the range of pericyclic reactions available to the synthetic organic chemist.