An explanation of the mechanism for high-temperature superconductivity is given, based upon a strong-coupling analysis of the extended Hubbard model previously introduced by one of us. The basic carriers are oxygen-hole quasiparticles in p\ensuremath{\sigma} orbitals, whose spin is strongly correlated with that of adjacent copper holes. These quasiparticles interact through the enhanced superexchange of the associated spins on the Cu sites, and an enhanced zero-point motion of the surrounding Cu holes. These are nonretarded attractive interactions whose strength increases as the oxygen-copper Coulomb repulsion increases and can be strong enough, for realistic parameters, to overcome the direct oxygen-oxygen Coulomb repulsion. The superconducting transition temperature that results is proportional to the Fermi energy of the oxygen holes.