Magnesium absorption has been studied in both humans and animals under diverse experimental conditions. As a result, the data often appear confusing and conflicting. In this review we attempt to summarize information concerning Mg absorption and, where possible, to reconcile apparently conflicting observations. Most studies suggest that Mg is absorbed predominantly in the distal intestine. At usual Mg intakes, Mg absorption occurs primarily by intercellular diffusional and solvent drag mechanisms. There is evidence for a saturable component of Mg absorption in the small intestine and the descending colon that is important at low dietary Mg intakes. Pharmacological doses of vitamin D increase Mg absorption in both vitamin D-deficient and vitamin D-replete animals. A substantial amount of Mg absorption, however, occurs independent of vitamin D. In addition, vitamin D may reduce Mg retention through increases in urinary Mg excretion. Intestinal interactions between Mg and calcium or phosphate have been demonstrated in both humans and animals. The nature of these interactions cannot be readily explained by data currently available.