Two types of models are presently used to describe the potential of a glass electrode: the ion exchange type, which is based on the equilibrium binding ratio of the glass surface, and the permeability type, based on the permeability ratio of the ions within the glass matrix. It is shown that the permeability type models can be derived from first physical principles while the exchange type models are questionable from the physical point of view. The derivation of the potential-concentration behavior shows that the steady-state characteristic of the electrode potential is determined by the selectivity properties of the dry glass layer. The rapidity of the potential response of the glass electrode can be traced back to the fact that, under certain constraints regarding the diffusion coefficients, the potential difference across this layer only depends on the boundary concentrations. The potential drift, in contrast, is determined by ion transport in the hydrated surface layers of the electrode. The theoretical predictions of single- and multi-layer permeability models are compared with experimental data for a sodium selective electrode.