This book explores chemical bonds, their intrinsic energies, andthe corresponding dissociation energies which are relevant inreactivity problems. It offers the first book on conceptual quantumchemistry, a key area for understanding chemical principles andpredicting chemical properties. It presents NBO mathematicalalgorithms embedded in a well-tested and widely used computerprogram (currently, NBO 5.9). While encouraging a "look under thehood" (Appendix A), this book mainly enables students to gainproficiency in using the NBO program to re-express complexwavefunctions in terms of intuitive chemical concepts and orbitalimagery.