This review summarizes recent advances and findings in the area of 'hidden' food allergens, i.e. allergenic foods that can either contaminate other foods, or be 'disguised' as part of a food, and cause allergic reactions. Newly emerging allergenic foods of increasing importance, recently developed methods for the detection of allergenic residues, the potential allergenicity of genetically engineered foods, and some unexpected sources of food allergens are described.