Adverse reactions to food resulting in gastrointestinal symptoms and due to immunologic reactions (allergy) are discussed: their pathogenesis, the prevalence of food allergens and the clinical digestive expressions of food allergy in children and adults are reviewed. In IgE-mediated food allergy, the usefulness of the biological available tests is considered, mainly CAP tests, for proceeding to the diagnosis and the monitoring of the allergic disease. Finally, the best actual diagnostic tools in food allergy are considered (clinical history, skin tests, biological tests and food oral challenges), with their limitations and indications.