A hybrid method is developed for the study of electronic states of localized defects in insulators. In this method, the inner shells of an atom (or ion) are represented by the first few orders of the ion size terms, while the outmost s and p shells are treated exactly as in the extended ion model. By the use of floating 1-s Gaussians as basis functions for the defect states, it is shown that the various terms (Coulomb, exchange and overlap) for the outermost s and p shells can be accurately represented by simple interpolation forms. This hybrid method retains the essential features of the extended ion model, but is much more efficient to apply. The method is first applied to study the electronic structure of the atomic-type self-trapped excitons in solid Ne. The results are in good agreement with experiment. The stability of the electron-bubble state in solid Ne is examined by the same method. Electronic states of F-centers in alkali halides are also studied. In this case, the effect of core wavefunctions on the defect states is examined by employing two different sets of core wavefunctions in the calculation: (1) free ion core wavefunctions and (2) core wavefunctions of ions in the crystal. The latter set of core wavefunctions is found to give energies in better agreement with experiment. A preliminary study of the self-trapped excitons in alkali halides is also reported.