Abstract: The important pericyclic reactions and Woodward-Hoffmann rules are presented by these lecture notes. The major viewpoints related to the rules of pericyclic reactions, such as the conservation of orbital symmetry, the frontier orbital approach and the conception of "aromatic transition state", are all discussed in some detail. The recent progress of concerted reactions, for instance as the quantitative and semi-quantitative treatment of the activity as well as regiose lectivity, is also surveyed in the present lecture notes.