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This review explores the contemporary definition of the term ‘balanced crystalloid’ and outlines optimal design features and their underlying rationale.Crystalloid interstitial expansion is unavoidable, but also occurs with colloids when there is endothelial glycocalyx dysfunction. Reduced chloride exposure may lessen kidney dysfunction and injury with a possible mortality benefit. Exact balance from an acid–base perspective is achieved with a crystalloid strong ion difference of 24 mEq/l. This can be done simply by replacing 24 mEq/l of chloride in 0.9% sodium chloride with bicarbonate or organic anion bicarbonate substitutes. Potassium, calcium and magnesium additives are probably unnecessary. Large volumes of mildly hypotonic crystalloids such as lactated Ringer's solution reduce extracellular tonicity in volunteers and increase intracranial pressure in nonbrain-injured experimental animals. A total cation concentration of 154 mmol/l with accompanying anions provides isotonicity. Of the commercial crystalloids, Ringer's acetate solution is close to balanced from both acid–base and tonicity perspectives, and there is little current evidence of acetate toxicity in the context of volume loading, in contrast to renal replacement.The case for balanced crystalloids is growing but unproven. A large randomized controlled trial of balanced crystalloids versus 0.9% sodium chloride is the next step.