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Purpose of Review: Numerous micronutrients are involved in antioxidant and immune defence, while their blood concentrations are frequently low in critically ill patients: this has fuelled many supplementation trials. Numerous observational, randomized studies have been published, which are presented herein., Recent Findings: Micronutrient concentrations must be analysed considering the context of the inflammatory response in critical illness. Low levels do not always indicate a deficiency without objective micronutrients losses with biological fluids. Nevertheless, higher needs and deficiencies are frequent for some micronutrients, such as thiamine, vitamins C and D, selenium, zinc and iron, and have been acknowledged with identifying patients at risk, such as those requiring continuous renal replacement therapy (CRRT). The most important trials and progress in understanding have occurred with vitamin D (25(OH)D), iron and carnitine. Vitamin D blood levels less than 12 ng/ml are associated with poor clinical outcomes: supplementation in deficient ICU patients generates favourable metabolic changes and decreases mortality. Single high-dose 25(OH)D should not be delivered anymore, as boluses induce a negative feedback mechanism causing inhibition of this vitamin. Iron-deficient anaemia is frequent and can be treated safely with high-dose intravenous iron under the guidance of hepcidin to confirm deficiency diagnosis., Summary: The needs in critical illness are higher than those of healthy individuals and must be covered to support immunity. Monitoring selected micronutrients is justified in patients requiring more prolonged ICU therapy. Actual results point towards combinations of essential micronutrients at doses below upper tolerable levels. Finally, the time of high-dose micronutrient monotherapy is probably over., (Copyright © 2023 Wolters Kluwer Health, Inc. All rights reserved.)