Plasma electrolytes, which include sodium, potassium, chloride, calcium and magnesium, are among the most frequently measured blood analytes in clinical medicine. All are measured by ion-selective electrodes (ISE). ISEs are present not only in clinical laboratory instrumentation but also in blood gas analyzers and other point-of-care measuring devices, allowing real-time monitoring of plasma electrolyte concentrations at the bedside. In a wide-ranging review the authors describe and explain the errors in electrolyte measurement that can occur with the use of ion-specific electrodes. The article begins with a brief introduction to the technology that highlights the essential difference between direct ISE used in point-of-care analyzers and indirect ISE used only in laboratory-based instruments. Among the many topics discussed are potential electrode instability due to sample matrix, potential interference with analysis due to prescribed drugs and blood transfusion, and preanalytical errors caused by inappropriate sample collection and handling technique. Some errors are analyte-specific and are addressed under their own heading so, for example, under the heading of potassium there is discussion of the many causes of pseudohyperkalemia (hemolysis, anticoagulant (EDTA) contamination, inappropriate sample storage conditions, etc). With 117 references this is a useful and comprehensive review of the many mechanisms that can give rise to erroneous patient electrolyte values, of relevance to both laboratory and clinical staff.