A complex relationship links magnesium and cancer. The aim of this review is to revisit current knowledge concerning the contribution of magnesium to tumorigenesis, from transformed cells to animal models, and ending with data from human studies. Cultured neoplastic cells tend to accumulate magnesium. High intracellular levels of the cation seem to confer a metabolic advantage to the cells, contribute to alterations of the genome, and promote the acquisition of an immortal phenotype. In magnesium-deficient mice, low magnesium both limits and fosters tumorigenesis, since inhibition of tumor growth at its primary site is observed in the face of increased metastatic colonization. Epidemiological studies identify magnesium deficiency as a risk factor for some types of human cancers. In addition, impaired magnesium homeostasis is reported in cancer patients, and frequently complicates therapy with some anti-cancer drugs. More studies should be undertaken in order to disclose whether a simple and inexpensive intervention to optimize magnesium intake might be helpful in the prevention and treatment of cancer.