Micronutrients, which include vitamins and minerals, are an essential component of any healthy diet. Though the body only requires small amounts of micronutrients, they nonetheless play a critical role in health and disease. The first chapter of this book includes a review of the relationships between micronutrient levels and clinical outcomes in patients with chronic kidney disease and the effects of different types of renal replacement therapy on micronutrient levels. The second chapter focuses on the use of micronutrient phytochemicals for optimizing health in normal subjects to assure they have the best possible immune response to an adverse disease event. Finally, the third chapter summarizes the data on the possible benefits and harms of vitamin supplements and essential minerals in pediatric ICU patients.