Modem valence-bond (VB) theory, in its spin-coupled (SC) form, is used to investigate the electronic structure of cyclooctatetraene at its lowest-energy tub-shaped D-2d geometry, as well as at two idealized planar geometries : a D-8h regular octagon and a D-4h octagon with alternating carbon-carbon bond lengths. It is found that at the D-2d geometry the eight SC orbitals comprising the active space represent equivalent, mostly pi-like, s(x)p(y) (x much less than y) hybrids which are engaged in four identical, largely independent, olefinic carbon-carbon double bonds. At this geometry, as well as at the planar D-4h geometry, cyclooctatetraene is definitely nonaromatic, which is indicated in the SC model by the absence of any significant resonance : The spin function for the valence electrons is dominated by one spin-coupling scheme involving four singlet electron pairs across the shorter "double" bonds. The SC description of the highest-symmetry conformation of cyclooctatetraene is compared to SC results for benzene and square cyclobutadiene in an attempt to establish the common destabilizing factor in antiaromatic systems. The analyses of the SC wave functions for C4H4, C6H6, and C8H8 suggest that the reason for the lower stability and higher reactivity of antiaromatic systems is related to a simultaneous unfavorable coupling of the spins of all valence orbitals to triplet pairs, which discourages bonding interactions and indicates biradical character.