Aim of the work:  there is an increased interest in the role of magnesium ions in clinical medicine, nutrition and physiology. Magnesium affects many cellular functions, including transport of potassium and calcium ions and modulates signal transduction, energy metabolism and cell proliferation. Magnesium deficiency is not uncommon among the general population: its intake has decreased over the years. Magnesium derangement results in various symptoms and signs; magnesium supplementation or intravenous infusion may be beneficial in various diseased states; so this review aimed to highlight the physiology of magnesium in humans, the derangement of magnesium in the form of hyper and hypomagnesemia , their clinical picture and the clinical and therapeutic uses of magnesium in the critical ill patients  . Methods: references were obtained from Medline, Google Scholar and Ovid from 1960 to 2017. All categories of articles (clinical trials, reviews, or metaanalyses) on this topic were selected. Conclusion: magnesium is a critical physiological ion; it has many known indications in anesthesiology and intensive care because of its interactions with drugs used in intensive care. Intensive care specialists need to have a clear understanding of the role of this important cation. Magnesium is gaining recognition as a clinically important electrolyte in intensive care and emergency medicine .Recent clinical trials and case reports increase interest of magnesium as an effective therapeutic agent for potentially life-threatening problems such as torsade de pointes, digitalis toxicity, bronchospasm and alcohol withdrawal, subarachnoid hemorrhage, acute myocardial infarction, preecalmsia , eclamsia ,hypertension , diabetes , metabolic syndrome and cardiac arrhthymias  .