Patch testing consists of the application of substances to the skin for the purpose of detecting specific hypersensitivity. It serves as an admirable diagnostic tool in the clinical study of patients who have inflammatory dermatitic reactions of an allergic nature. It allows a rapid and relatively safe review of the tolerance of the skin to a large number of materials known to cause reactions in the sensitized person. Although disarmingly simple in principle, it does require skill in execution and interpretation. It is only one cog on the diagnostic wheel! Furthermore, one must recognize that patch testing involves the deliberate reproduction of disease and as such must be employed with discretion. Admittedly the disease is miniaturized down to a patch, but nonetheless it is iatrogenic. Hence, the responsible physician will generally limit patch testing to those circumstances where the history, the localization, and a thoughtful review of the patient's contactants