The analytical value of terms used to assess the performance characteristic of ion-selective electrodes (ISE) such as selectivity coefficient, lower limit of detection, response time, and life time depends on the constancy of these parameters. The response time of the indicator electrode and that of the other parts of the electrochemical cell cannot be estimated separately. Since response time data dramatically increase with the decrease of the preselected differential quotient, the practical differential quotient values must be selected according to the required analytical accuracy. Response times ranging from a few milliseconds to some minutes were published for silver halide-based electrodes of the second kind, for liquid ion-exchanger-based nitrate and calcium ISE, for cation-selective glass electrodes and pH-sensitive glass electrodes, as well as for various types of precipitate-based electrodes. The measured response time data corresponding to different definitions can be interrelated if the respective mathematical models are known.