In the spin-coupled description of molecular electronic structure, an N-electron system is described byNdistinct—but non-orthogonal—orbitals, whose spins are coupled to the required resultantSin all possible ways. The coefficients of the basis functions comprising the orbitals and the coefficients of the different spin functions are fully optimized. The orbitals are frequently highly localized, and hence the model incorporates considerable electron correlation while retaining a high degree of visuality. The spin-coupled wave function is refined by non-orthogonal configuration interaction, and the final wave functions are of high quality but very compact. The various aspects of this theory are illustrated by a series of examples of increasing complexity: the H2molecule, the BeH molecule, the3B1andlA1states of CH2and the cycloaddition of CH2to ethenes, the 7t-electron system of benzene, and diazomethane (CH2N2). The results provide clear descriptions of the electronic structure and the associated processes. In benzene, the six π orbitals are highly localized, with far-reaching implications for the description of aromatic systems. The case of diazomethane shows that the central N atom takes part infiv. electron-pair bonds, and the same is true for a series of molecules such as N2O, Hcno, NO2, and CH2NHO (nitrone), whose structures have long caused problems in valency theory.