  The quantum Coulomb glass model describes disordered interacting electrons on
the insulating side of a metal-insulator transition. By taking quantum
fluctuations into account it can describe not only the localized limit but also
the weakly localized regime. We discuss several possibilities to generalize the
concept of Anderson localization to interacting electron systems such as the
quantum Coulomb glass and define criteria for localization. The corresponding
physical quantities are calculated by numerically exact diagonalization. The
results indicate that single-particle excitations close to the Fermi energy
become more strongly localized under the influence of interaction.
