Abstract: A knowledge of the chemical composition of blood, urine and other body fluids is a daily requirement for departments of biochemistry. Electronic circuitry and computers to process the data are readily available, but satisfactory transducers to convert chemical composition into electrical signals are frequently the weakest link in the chain of measurement. This review is concerned with one group of transducers: ion-selective electrodes. Since the commercial success of the calcium and fluoride ions-selective electrodes in the mid-sixties, a range of other electrodes has become available. Their use has already conferred considerable benefits upon medicine, e.g. rapid, low-cost, multiple assay of major blood components; diagnostic surveys of chloride sweat levels relating to cystic fibrosis; and monitoring blood fluorides during and after halothane administration. Their adoption for indirectly sensing enzymes and associated substrates is particularly noteworthy. Recent advances in electronics, coupled with flow injection schemes based on ion-selective electrodes, have facilitated the management of hundreds of samples daily. However, developments in the mechanistic knowledge of these sensors have not matched the increase in their application, although definite progress can be reported; for example with regard to the origin of the potential signals induced by ion activities in solution. Numerous techniques have been devised to unravel mechanistic problems, among which radioisotope tracer and impedance measurements may be cited as especially valuable. Selectivity performance, particularly in complex biological media, and undesirable features such as protein poisoning, need further research. Organic chemists are now better placed to synthesize new designs of acyclic and cyclic molecules as mobile site, ion-selective, sensor materials which, with appropriate mediator solvents, provide improved sensor cocktails. This design feature is well illustrated by the continuing guest for a lithium ion-selective electrode compatible with the high levels of sodium interference in blood.