Magnesium (Mg) is an essential ion for life and is involved in many important biological processes. Mg deficiency may have serious consequences and has been implicated in many diseases. Assessment of Mg status is difficult because Mg is essentially intracellular and there is no simple, rapid and accurate laboratory test to determine total body Mg. New, non-invasive techniques are being developed that could be used in the near future. Mg therapy is clearly indicated in "torsade de pointes" and preeclampsia. Mg therapy is not recommended in the treatment of acute myocardial infarction, stroke or non-severe acute asthma. Nevertheless, Mg administration may be useful to reduce cerebral ischemic events after aneurysmal subarachnoid hemorrhage or in the treatment of severe acute asthma, specifically in children. Finally, the potential post-operative analgesic effect of Mg is still debated and results from new, larger clinical trials may help clarify this issue. This article reviews physiological functions and pharmacology of Mg status as well as the causes, clinical manifestations, and treatment of Mg deficiency. The possible analgesic effects of Mg in the postoperative period are also reviewed.