Pericyclic reactions are a significant part of organic chemistry, usually introduced with only a few of the most important examples, early in an undergraduate's career - everyone has to know of the existence of some of these reactions. They are dealt with properly, almost everywhere in the world, in the third or fourth year, or sometimes as part of a first year course for graduate students. At this stage, a wider range of reactions is introduced, and the governing principles explained. This book does that.