Magnesium is the second most abundant intracellular cation. It is essential for a wide variety of metabolically important reactions, in particular those involving ATP. Hypomagnesaemia is surprisingly common in hospital populations but is sometimes either undetected or overlooked. Serum magnesium concentrations provide a guide to magnesium status but while hypomagnesaemia is a reliable indicator of magnesium deficiency, normomagnesaemia does not exclude magnesium depletion. A wide variety of conditions predispose to magnesium depletion. Clinical magnesium deficiency has potentially fatal consequences in vulnerable groups of patients and should be excluded in all such cases. Magnesium deficiency may result in hypokalaemia, hypocalcaemia or other disturbances of electrolyte homeostasis, refractory cardiac arrhythmias, or increased sensitivity to digoxin. The capacity to measure serum and urine magnesium concentrations rapidly, regularly and reliably should be part of the repertoire of all clinical chemistry laboratories involved in the care of critically ill patients.