Antibonding orbitals themselves play a central role in describing chemical reactivity of one molecule with another through donor-acceptor interactions between a high-lying occupied of one species with a low-lying antibonding orbital of the other. Frontier orbital theory is constructed on these ideas. Antibonding orbitals also play an important role in describing strong electron correlations. A simple example is the stretching of the H-H bond which leads, in a minimal basis, to a strong increases in the amplitude for σσ → σ σ excitation which breaks the bond.