Electrochemical principle of ion-selective electrodes (ISEs) based on the ion-transfer reactions across a polarizable organic or oil/aqueous or water interface is described; the amperometric ISE and the potentiometric ISE are addressed. Electrochemical sensors and biosensors based on amperometric ISEs are discussed in some details. Amperometric sensors for monitoring ammonia (and other volatile amines) can be constructed on the basis of amperometric ammonium-ISE, where the pulse amperometric technique can successfully be employed. Urea and creatinine biosensors can also be fabricated by immobilizing urease or creatinine deiminase, respectively, on the surface of the amperometric ammonia sensor. Some favored characteristics of amperometric ISE-based sensors and biosensors are discussed with reference to the sensors and biosensors described.