The present and prospective rapidly increasing usefulness of good LCAO-MO calculations on the electronic states of simple molecules in the interpretation of observed excited states is pointed out. As examples, the observed and predicted states of, in particular, the π3π configurations of N2and CO are compared with those of C2H2and HCN and with those of CO2and CS2. Further, the results of LCAO-SCF calculations on CO2and on O3are surveyed, and it is shown how these can be helpful in interpreting and understanding the ground and excited states of AB2(expecially AO2) molecules in general. A new interpretation of the so-calledd3П state of CO as a casebπu3πg,3Δustate is proposed. Tentative interpretations of some of the ultraviolet absorption spectra of C2H2, HCN, and of a number of AB2molecules are reviewed or suggested, including some discussion of the shapes of excited states. The AB2discussion is a revision of one given earlier. Finally, following up a suggestion of Coon, it is pointed out that there exists strong evidence for slightly unequal A—O distances in certain excited states of C1O2and SO2