Immunologically mediated reactions to foods are responsible for up to 10% of all allergic syndromes in dogs and cats. Skin lesions (pruritus, erythema and papules) represent the main manifestation (70 to 80%) compared to only 10 to 15% incidence of gastrointestinal signs in combination with skin problems or alone. Diarrhea, vomiting, low appetite, chronic weight loss, abdominal pain and lethargy are the most common signs involved in gastrointestinal food allergy. There exists no breed or sex predilection in dogs or cats; signs may occur at any time during life time. The immunological nature of the disease can only be guessed at from anamnestic and clinical features and must be verified by presence of lymphocytes, plasma cells, mast cells and eosinophilic granulocytes in histological specimens. At this time, the responsible allergen can only be identified in a restriction diet trial based on food which the animal has not been fed before, given exclusively for at least four weeks to the pet. Changing from one brand of commercial diet to another is not recommended. The diagnosis is conclusively proven by reproducing the symptoms by feeding the original diet after the elimination of signs on the new diet. The pet can then be fed on a commercial diet (or home-made food) without the offending allergen(s).