Carnitine performs a critically important role in energy metabolism and is synthesized in the healthy adult predominantly in the liver and kidney. The typical well balanced American diet contains significant amounts of carnitine as well as the essential amino acids and micronutrients needed for carnitine biosynthesis. Thus carnitine is an infrequent problem in the healthy, well nourished adult population in the United States. However, carnitine can be a conditionally essential nutrient for several different types of individuals. Preterm infants require carnitine for life-sustaining metabolic processes but have a carnitine biosynthetic capability that is not fully developed. There is an increasing number of documented problems with carnitine metabolism in preterm infants not receiving an exogenous source of carnitine indicating that endogenous biosynthesis of carnitine is not adequate to meet the infant's need. Children with different forms of organic aciduria appear to have a greatly increased need for carnitine to function in the excretion of the accumulating organic acids. This need exceeds their dietary carnitine intake and carnitine biosynthetic capability. Renal patients treated with chronic hemodialysis appear to lose carnitine via the hemodialysis treatment, and this loss cannot be repleted simply by endogenous biosynthesis and dietary intake. Treatment with drugs such as valproic acid and metabolic stresses such as trauma, sepsis, organ failure, etc, can also result in a requirement for exogenous carnitine. Accurate assessment of the carnitine status of patients at risk for carnitine deficiency is fundamental to the identification of those patients who require carnitine as the result of altered metabolism.