The main features of the ion permeability of gramicidin channels are summarized. The significance of maximums in the single channel conductance-concentration curves, of concentration-dependent permeability ratios, and or current-voltage curves with concentration-dependent form, as well as of other features, is discussed in terms of the mechanism of the ion transfer processes. The observations are then shown to be accounted for by rate theory expressions derived for a model pore consisting of two sites in series and in which ions are not permitted to pass each other. The status of other models is briefly reviewed.