1 1 orbita1 theory has had some notable successes in the ana1ysis of individua1 organic reactions and in corre1ations between reaction series. Gen- ll? the theory has invoked to exp1ain known chemica1 phenomena, and rather infrequent1y [ broad1y-based predictions. In 1965 Woodward and published ? series of papers the 1 1 orbita1 inter- pretation of various types of concerted cyc10-addition reactions, which hitherto had rather 1? understood. Because these processes (now known as pericyclic reactions) had great synthetic importance, and because the Woodward-Hoffmann theory was stated so explicit1y as to allow [ usefu1 and far-reaching predictions to made, the genera1 acceptance of the so-called Woodward-Hoffmann Ru1es was very rapid. Judging from the vast number of publications that have appeared, ? great dea1 of experimentaleffort has channelled into this genera1 area since that time, the results of which provide ? vindication of the ru1es. The theoretica1 basis of Woodward and Hoffmann's method has, however, the subject of criticism and controversy, and ? of alternative theoretica1 methods have a1so appeared. university departments (inc1uding?ur own) have for some time covered pericyclic reactions in their undergraduate and graduate courses. Because aims, teaching methods, and persona1 preferences differ wide1y, each of the various theoretica1 methods have achieved some currency. We have sought to?1 these methods in some sort of perspective. The book is intended to introductory, being aimed primarily at fina1 year undergraduates and first year postgraduates.