Glutamine is the most abundant free amino acid in the circulation. It is a primary fuel for rapidly dividing cells and plays a key role in the transport of nitrogen between organs. Although glutamine is absent from conventional regimens aimed at nutritional support, glutamine deficiency can occur during periods of metabolic stress; this has led to the reclassification of glutamine as a conditionally essential amino acid. Experiments with various animal models have demonstrated that the provision of glutamine can result in better nitrogen homoeostasis, with conservation of skeletal muscle. There is also considerable evidence that glutamine can enhance the barrier function of the gut. This review concludes by discussing the clinical evidence that supports the inclusion of stable forms of glutamine in solutions of nutrients.