In order to understand the mechanism of the ion-selective electrodes, it would be ideal to study the various reactions producing the electrochemical signal. Intensive research work on ion-selective electrodes started in the early 1960s. It was kept in mind that electrode studies under dynamic conditions may provide information on the partial processes of the electrode response mechanism. Consequently, in the second half of the 1960s, great hopes were attached to response time measurements using the activity step method. In the late 1970s and early 1980s, attention was given on a new aspect of the dynamic behavior of the electrodes, namely, the response in the presence of interfering ions. In all analytical applications, especially in flow analytical techniques, the knowledge of the dynamic behavior of ion-selective electrodes is of utmost importance. In the majority of cases, the time requirement of convective diffusion under flow conditions is commensurable with the response time of the sensors.