Electrochemical sensors are mainly based on potentiometry, amperometry, or voltammetry, i.e. they use which occur during electrochemical processes. The corresponding processes can be attributed to “classical inorganics”—we have already dealt with such systems in great detail in Parts I–III (we need only recall the silver/silver chloride electrode and the calomel electrode, or indeed the Clark electrode, to which we shall return in ► Sect. 14.1)—but biochemical or even “purely biological” processes can also be investigated amperometrically or voltammetrically, provided that an appropriate sensor system is available. We will turn to this subject in ► Sect. 14.2.