Reported concentrations for magnesium in breast milk vary over a wide range (15 to 64 mg/L) with a median value of 31 mg/L and 75% of reported mean concentrations below 35 mg/L. Constitutional variables such as adolescent motherhood, gestation length, maternal undernutrition, metabolic disorders (diabetes, galactosemia), race, stage of lactation, sampling techniques (foremilk and hindmilk), as well as environmental variables such as socio-cultural diversity, smoking habits, dietary calcium and magnesium (including supplementation), vegetarianism, calciotropic agents (immunoreactive calcitonin, vitamin D), medication (hormonal contraceptives, magnesium sulfate) are critically reviewed in relation to changes in milk magnesium concentrations. Magnesium secretion into breast milk does not seem to be affected by the studied variables.