The geometry and relative energy of the seven low-lying electronic states of ozone and the ground state of ozonide anion have been determined in C-2v symmetry by the complete active space self-consistent field (CASSCF) and the multireference Moller-Plesset perturbation (MRMP) methods. The results are compared with the photodetachment spectra of O-3(-) observed recently by Arnold et al. The theoretical electron affinity of ozone is 1.965 eV, which is 0.14 eV below the experimental result of 2.103 eV. The calculated adiabatic excitation energies !assignment of Arnold et al. in parentheses, of ozone are (3)A(2) 0.90 eV (1.18 eV), B-3(2), 1.19 eV (1.30 eV), B-3(1), 1.18 eV (1.45 eV). (1)A(2), 1.15 eV (similar to 1.6 eV), B-1(1), 1.65 eV (2.05 eV), and B-1(2), 3.77 eV (3.41 eV), respectively. Overall the present theory supports the assignment of Arnold et al. However, the simple considerations of geometry and energy are insufficient to determine a specific assignment of the B-3(2) and B-3(1) states. The dissociation energy of the ground state of ozone is computed to be 0.834 eV at the present level of theory. The present theory also predicts that none of the excited states lies below the ground state dissociation limit of O-3.