Using an extension of a first-principles method developed by King-Smith and Vanderbilt Phys. Rev. B, 49, 5828 1994 , we investigate the effects of in-plane epitaxial strain on the ground-state structure and polarization of eight perovskite oxides: BaTiO3, SrTiO3, CaTiO3, KNbO3, NaNbO3, PbTiO3, PbZrO3, and BaZrO3. In addition, we investigate the effects of a nonzero normal stress. The results ar...