This book is a collection of selected papers on the Frontier Orbital Theory by Nobel prizewinner Kenichi Fukui (Chemistry 1981), with introductory notes. It provides the basic concept and formulation of the theory, and the physical and chemical significance of the frontier orbital interactions in chemistry, together with many practical applications. The formulation of the Intrinsic Reaction Coordinate and applications to some simple systems are also presented. The aim of this volume is to show by what forces chemical reactions are driven and to demonstrate how the regio- and stereo-selectivities are determined in chemical reactions. Students and senior investigators will gain insight into the nature of chemical reactions and find out how quantum chemical calculations are connected with chemical intuition.