Magnesium (Mg) is a biologically essential mineral and Mg deficiency is known to lead to severe biochemical and symptomatic disorders. Radioactive isotopes and, more recently, stable isotopes have been used as research tools to determine intestinal Mg absorption in humans and animals under different nutritional and physiological conditions. Mg isotopes are given orally or orally plus intravenously and analysed in faeces and/or in plasma and urine in order to calculate intestinal Mg absorption and possibly endogenous Mg excretion. Mg isotopes have been used to assess exchangeable pools of Mg under nutritional and physiopathological conditions. Mg isotopes are given intravenously and are analysed in plasma and urine to calculate the size and half-life of the various Mg exchangeable pools. More recently, in vitro isotopic tests have been developed to study the need of cells for Mg in different nutritional and genetic conditions. Whole blood is incubated with Mg isotopes and isotopic blood cell enrichment is measured, which reflects the avidity of cells for Mg and thus its initial status. This paper is a report on the use of stable Mg isotopes and their advantages in these different fields of Mg absorption and metabolism. The studies available have clearly demonstrated that stable isotopes provide a useful research tool for determining intestinal Mg absorption, and represent a precious research tool for the study of Mg metabolism and the assessment of Mg status.