Immunonutrition is an emergent and interdisciplinary subject, since it comprises several aspects related to Nutrition, Immunity, Infection, Inflammation, and Injury or tissue damage, what is known as Nutrition and 4 “Is”. Within these interactions the endocrine, nervous and immune systems are involved, microbiota being a part of the last one. Nowadays, gut microbiota has been shown to play an essential role, not only in the gastrointestinal tract but also into the nervous system, because of its bilateral connection. There are several methods to study Immunonutrition, which allow measuring different immunological biomarkers to provide information about the nutritional status. However, it should be taken into account that there is not a single gold standard parameter to evaluate the cause- effect relationship between nutrition and the immune system. On the contrary, a combination of biomarkers have to be assessed depending on the different nutritional situations. Since Immunonutrition is a multidisciplinary matter as mentioned above, the study on the interactions between nutrition and the immune system has not been exclusively focused as such, but bearing in mind other systems of the organisms as well as a wide range of confounding factors and determinants coming from idiosyncratic features, genes and lifestyle of each individual. Therefore, Immunonutrition allows to study the following research fields: 1) Evaluation of nutritional status in presumably healthy people with risk of malnutrition (children, adolescents, adults, pregnant women, elderly, and sportspeople); 2) Assessment of the evolution and progress of patients with nutrition and immune-related diseases, such as food allergies, eating and metabolic disorders; 3) Evaluation of the effects of nutrients, bioactive compounds and both conventional and functional foods on the immune system; 4) Evaluation of impact of lifestyle determinants on the immune system, such as diet, food behaviour, physical activity, sedentariness, sleep quality and quantity, and as a key factor, stress.