It must be pointed out that the Nernst equation is clearly based on the assumption of thermodynamic equilibrium at the phase boundaries between membrane and outer solution. The mathematical formulation describing the transient functions differs according to the assumed rate-determining partial process of the overall electrode reaction. The transient functions of a galvanic cell incorporating the ion-selective indicator electrode can be affected by the parameters of the measuring electronics. In the case of membranes of high resistance, the transient function of the cell is determined primarily by the electrical measuring circuit. Several authors discussed critically the relative merits of various models interpreting the potential vs. time transient functions obtained by the activity step method. These studies revealed that the existing theories cannot describe the transient functions with satisfactory accuracy in the entire time domain. This fact is due partly to the mathematical complexity of the problem and partly to the differences between the experimental conditions and the model assumptions.