Monitoring and measuring magnesium values are essential to prevent the development of numerous complications in perioperative medicine and critically ill patients. Although current studies suggest that measuring free ionized magnesium (Mg++) is more useful for estimating magnesium status, clinicians often still rely on measurement of total serum magnesium (tMg) to determine if supplemental magnesium is needed. Electrolytes (Na+, K+, Ca++, Cl-) are all currently measured as ions because that is their only clinically active form. Now Mg++ can be measured in the same way. Just as ionized Ca (Ca++) is recognized as the standard for assessing Ca status, Mg++ is now being recognized as vital in assessing magnesium status, especially in critically ill patients. In this webinar we summarize recent literature to describe why it is better to measure Mg++, not tMg, when assessing magnesium status.