  We determine the zero-bias anomaly of the conductance of tunnel junctions by
an approach unifying the conventional Coulomb blockade theory for ultrasmall
junctions with the diffusive anomalies in disordered conductors. Both,
electron-electron interactions within the electrodes and electron-hole
interactions between the electrodes are taken into account nonperturbatively.
Explicit results are given for one- and two-dimensional junctions, and the
crossover to ultrasmall junctions is discussed.
