The repercussions of nutrition on the immunological system are described. The current situation is discussed with regard to the handling of the immuno-inflammatory response to aggression and the improvement of the immune response. The goal pursued by nutritional manipulation of the immune system is to improve the prognosis and reduce the complications in critical patients subjected to surgical or traumatic aggression or sepsis. The following points are reviewed: the importance of preserving the axis from the liver to the digestive tract, the advantages of particularly early enteral nutrition over parenteral nutrition, the substances currently considered as immunonutrients and the available clinical evidence on their use, with a series of conclusions for current decisions and future developments.