Appropriate micronutrient therapy, both to meet physiological requirements as well as treat disease-related deficiencies, is of vital importance in critical care. It is important to recognize the individualized needs of the ‘critically ill patient’ and tailor both overall medical and nutritional management plans. Existing literature on this population group is limited by factors such as patient heterogeneity, sample sizes, and variability in baseline nutritional assessment. It is therefore important to ensure continuous evaluation of individual nutritional needs as there may be considerable departure from general recommendations. A feature of the “critically ill patient” is the association with a catabolic stress state and the influence of the systemic inflammatory response. This is coupled with adjustments in requirements for macronutrients as well as micronutrients, secondary to altered physiological states such as infection and multi-organ dysfunction. The purpose of this chapter is to discuss and highlight relevant micronutrient and vitamin physiology and outline how these are altered in the critical care setting. The aim in this setting is to enable an adequate supply of micronutrients to ensure homeostatic, metabolic, and immune functions to optimize recovery with respect to the considerations below.