Magnesium, Mg²⁺, is the second most abundant intracellular cation after potassium and the fourth most abundant in the body. It was first isolated in 1808 by the English chemist, Sir Humphrey Davy. Magnesium is essential to numerous biochemical reactions. It modulates key physiological processes such as metabolic biochemistry, nucleic acid synthesis, receptor-binding and ion flux. The western diet falls short of the recommended daily allowance of 4.5 mg/Kg/day and important dietary sources are seeds, grains, nuts and green vegetables. It is used as a therapeutic agent in a broad range of pathologies: neurological, cardiovascular, respiratory, gastrointestinal and obstetric. The pharmacokinetics and pharmacodynamics of magnesium, as a drug, are not well understood. Despite its fundamental importance to human physiology, it remains an electrolyte that is not routinely measured as part of the “urea & electrolytes” test and is the most overlooked electrolyte deficiency in hospital inpatients. This review will summarise the importance of magnesium homeostasis, its pharmacological effects and clinical applications.