Synopsis-The difficulties of immunological nonienclature are discussed, the term ALLERGY defined and the various types of HYPERSENSITIVITY reactions are listed and characterized. Evidence for the association of Type I and Type 11 hypersensitivity reactions with COSMETICS is discussed. A table of cosmetic ingredients which have been implicated as SENSITIZERS are given. PREDICTIVE PATCH TESTS for contact sensitizers on GUINEA-PIGS and man are evaluated. The difficulties of testing for ALLERGENS likely to produce Type I hypersensitivity are discussed. IN VITRO tests for sensitizers are mentioned. The failure of all standard tests in the detection of weak sensitizers is emphasized.asized.