This article begins with an introduction to ion‐selective electrodes (ISEs). The classification protocol based on the most recent International Union of Pure and Applied Chemistry (IUPAC) recommendations is given. The historical development of ISEs is summarized, starting with the pH electrode. For the inorganic cations and anions, a periodic table “time‐line” is given. Recent history has seen the rise of polymer membrane ISEs, which are important in clinical analysis. The historical section concludes with a brief look at these ISEs. The general theory of the potential generation process is explained, and then aspects of the potential generation process that are specific for the different classes of ISEs. A short review of the instrumentation needed to make the measurements is given, followed by an overview of the latest commercially available instrumentation. One aspect of ISE research is the development of selective probes for determining the analyte content of individual cells. This requires miniaturization, and this has been achieved by the development of microelectrode ISEs. An overview of this area is given. Finally, some recent developments that represent the directions of research in the ISE field are given. Included are the development of sensors for patient monitoring, the development of ISE sensor arrays, and the development of more sophisticated electronic and mathematical methods of data analysis.