Predictive tests are of value in forecasting the response of a population to a sensitizer; diagnostic testing is used to determine what substances may actually be producing dermatologic problems. Skin sensitization predictive and diagnostic data for the eleven most frequently encountered skin sensitizers in Western Europe, Canada and the United States are reviewed. These compounds include two drugs (benzocaine and neomycin), two cosmetic ingredients (p-phenylenediamine and balsam of Peru), four preservatives (formaldehyde, ethylenediamine, parabens and mercurials) and three ingredients of wearing apparel (nickel, chromium and thiram). Many of the data were collected by the North American Contact Dermatitis Group and the International Contact Dermatitis Research Group on tests with 1,200 and 4,825 dermatologic patients, respectively; the remainder were obtained by individual investigators with smaller groups of subjects. The data obtained by various investigators are discussed in relation to the factors which affect the extent and degree of sensitization which they can cause.