Ever since the classic The Conservation of Orbital Symmetry by Woodward and Hoffmann, there has been a need in the publication of many books and excellent review articles dealing with this topic. Now, focus had shifted to uncover the mechanisms of the so-called “no-mechanism reactions.” The uncovering of the fact that orbital symmetry is conserved in concerted reactions was a turning point in our understanding of organic reactions. It is possible to predict the stereochemistry of such reactions by following the simple rule that stereochemical consequences of reactions of reactions initiated thermally will be opposite to those performed under photochemical conditions. Study of pericyclic reactions, is an integral part of our understanding of organic reaction mechanisms. The book opens with an introduction (chapter 1), which, besides providing background information needed for appreciating different types of pericyclic reactions, outlines simple ways to analyse these reactions using orbital symmetry correlation diagram, frontier molecular orbital (FMO), and perturbation molecular orbital (PMO) methods. Electrocyclic, sigmatropic, and cycloaddition reactions are subsequently in chapter 3. Chelotropic and 1,3-dipolar cycloaddition reactions as examples of concerted reactions are also discussed in chapter 3. Aromaticity of benznoid and benzanoid compounds, Frost Muslin diagrams are discussed in chapter 4. Assuming that a student has taken courses in organic chemistry that include reaction mechanisms and stereochemistry, The book is meant to be taught as a one-semester course to graduste and senior undergraduate students majoring in chemistry. One has to remember that a book designed for a one-semester course cannot include all the reactions reported in the literature.