This volume contains review and research articles in one particular ar ea of quantum chemical methodology: the theory of electron correlation and the localization problem. Correlation energy (i.e., the energy be yond the Hartree-Fock) plays an important role in the theoretical desc ription of several chemical processes (reactions, excitations, ionizat ion) while localization in quantum chemistry usually emerges as a math ematical transformation of the wave function. The connection between t hese two issues is provided by the need to evaluate correlation energy for larger molecules, which is...