Food allergy is an untoward immunologic reaction to food. The number of conditions that need to be considered in the differential diagnosis is large and includes adverse reactions to foods that do not have an immunologic basis. Intestinal immune elements are an integral feature of the intestinal mucosa and, besides modulating gastrointestinal function, represent a significant part of the host's overall immune system. Despite increasing knowledge, the precise disturbances in immune function that lead to the appearance of food allergy are still unclear. Food allergy will remain primarily a clinical diagnosis, relying on history and response to dietary elimination and rechallenge until more specific tests become available.