There are three groups of patients with abnormalities of magnesium homoeostasis. The first one include patients with magnesium deficiency (low total body magnesium content) and a resultant hypomagnesaemia (low serum magnesium concentration). Patients with hypomagnesaemia (serum magnesium concentration <0.75 mmol/L) in the absence of magnesium deficiency (i.e., a normal total body magnesium content) consist the second group. The third group include patients with magnesium deficiency (low total body magnesium content) but no evidence of hypomagnesaemia (i.e., a normal serum magnesium concentration). Magnesium deficiency can be caused by decreased magnesium intake from the diet, decreased magnesium absorption, or increased renal magnesium excretion (renal magnesium wasting). The narrative review examines the causes, clinical and laboratory signs of magnesium deficiency in the body, and the effect of magnesium supplementation on health indices. Groups of people who are more likely to suffer from magnesium deficiency are outlined. Emphasis is placed on the use of questionnaires to identify individuals with possible magnesium deficiency. The changes on the electrocardiograms that are characteristic of the initial magnesium deficiency and that occur in case of its increase are given. The effect of additional intake of magnesium salts on blood pressure changes in individuals with and without baseline hypertension is discussed in detail. Factors that make it difficult to assess the relationship between magnesium intake and abnormalities in lipid and carbohydrate metabolism are listed. An association between increased dietary magnesium intake and reduced risk of certain cardiovascular diseases, diabetes mellitus, and overall mortality has been demonstrated. Keywords: hypomagnesaemia, blood pressure, diabetes mellitus.