In 1962, Professor John D. Roberts wrote a book on molecular orbital calculations for chemists that was very readable. This book is based on that relevant work. Senior undergraduate and beginning graduate students, as well as working bench chemists who want to gain conceptual skills for comprehending chemical processes, are the target audience for this book. Although ab initio and more sophisticated semi-empirical MO approaches are thought to be more accurate than HMO in a strict sense, there is strong evidence that HMO can yield accurate relative results, especially when comparing closely related molecular species. HMO can be used to understand aromaticity and the geometry of simple molecular orbitals, as well as the electronic structure of 1t-systems. Still interested in subtle electronic interactions for interpretation of UV and photoelectron spectra.