The aim of this paper is to discuss, on the basis of an extensive literature review, the role of magnesium in health and disease. Magnesium is an essential cation playing a crucial role in many physiological functions. It is critical in energy-requiring metabolic processes, in protein synthesis, membrane integrity, nervous tissue conduction, neuromuscular excitability, muscle contraction, hormone secretion, and in intermediary metabolism. Serum magnesium concentration is maintained within a narrow range by the small intestine and kidney which both increase their fractional magnesium absorption under conditions of magnesium deprivation. If magnesium depletion continues, the bone store helps to maintain serum magnesium concentration by exchanging part of its content with extracellular fluid. The abundance of magnesium within cells is consistent with its relevant role in regulating tissue and cell functions. Recent data suggest that large fluxes of magnesium can cross the cell plasma membrane in either direction following a variety of hormonal and non-hormonal stimuli, resulting in major changes in total and, to a lesser extent, in free magnesium content within tissues. Imbalances of magnesium are common and are associated with a great number of pathological situations responsible for human morbidity and mortality. A large part of the population may have an inadequate magnesium intake, and in particular elderly subjects and athletes may be prone to chronic latent magnesium deficiency. Magnesium deficit is frequently observed in alcoholics and diabetic patients, in whom a combination of factors contributes to its pathogenesis. We will discuss some of the aspects of the involvement of magnesium in the etiology of some pathological situations, such as cardiovascular diseases, diabetes, pre-eclampsia, eclampsia, sickle cell disease and chronic alcoholism.