ObjectiveTo provide a brief overview of the causes, diagnosis, and therapies of electrolyte abnormalities frequently seen in critically ill children.Data SourcePublished articles and bibliographies identified from pertinent review articles and books published in the past 20 yrs on fluid and electrolyte therapy in children.ConstraintThe review includes only human studies and articles published in the English language.Study SelectionHuman studies pertaining to abnormalities of water homeostasis, sodium, potassium, calcium, and magnesium homeostasis in children were selected. Prospective randomized studies were selected as much as possible.Data ExtractionPertinent data abstracted from multiple, independent clinical studies were used to develop an approach to electrolyte abnormalities in critically ill children.Data SynthesisAn approach to diagnosis and therapies of common electrolyte abnormalities in critically ill children.ConclusionsElectrolyte abnormalities are common in critically ill children and can be easily treated once recognized. Further studies are needed to better understand the role of ionized calcium and magnesium in neonatal and pediatric critical illness.