Adverse reactions to food and food additives are defined by its different pathomechanisms. Clinically most important are allergic (immunologic) reactions (types I and III), which occur in 7-10% of the general population, and nonimmunologic (pseudoallergic) reactions (PAR), which occur in 1-2%, besides nonallergic reactions by vasoactive amines. Clinical features are equal. Sources of antigens for allergic reactions are proteins of cow's milk, fish, hen's egg, meat, and all kinds of cereals, fruits, vegetables, and spices, while pseudoallergic reactions are induced by chemicals (preservatives, colorants, antioxidants). The diagnostic procedures of allergic reactions include the proof of sensitization by case history, skin test, specific IgE (and IgG) in-vitro, elimination diet and provocation test; pseudoallergic reactions with no underlying sensitization are diagnosed only by elimination and provocation procedures.