At present, neutral-carrier- and ¡on-exchanger-based ion-selective liquid membrane microelectrodes are available for the measurement of H+, Li+, Na+, K+, Mg2+, Ca2+, NH4+and CI-. Fundamental aspects, design features and limitations of microelectrodes are discussed and examples for their application in physiology are given. The most important characteristics are summarized in view of intra- and extracellular physiological measurements. The selectivity coefficients KijPolwith respect to the most common ions are compared to calculated values Kij,maxPol, required for measurements in intra- and extracellular samples without significant interference. For intracellular microelectrode studies, these required values are fully met for H+, K+, Mg2+, Ca2+and CI-, while in the case of extracellular measurements H+, Na+, K+and Ca2+can be assayed adequately.