The theory of the HMO total π-electron energy (E) of benzenoid hydrocarbons is surveyed with particular emphasis on the research of its dependence on molecular structure. Identities, bounds and approximate formulas for E are considered. The dependence of E on the size of the molecule and on the number of Kekule structures is discussed in detail. The effect of cycles on E, and six-membered rings in particular, is considered within the framework of the theory of cyclic conjugation.