Patch testing is a standardized, in vivo diagnostic test for type IV hypersensitivity reactions, resulting in allergic contact dermatitis, which clinically resembles eczema. Common allergens include fragrance chemicals, hair dyes, metals, rubber accelerators and preservatives. Known allergens at particular concentrations in optimal vehicles are tested on the upper back under occlusion for 2 days. Readings according to international criteria are usually performed on days 2 and 4. Irritant reactions can closely resemble allergic ones, and further tests may be necessary to discriminate. Interpretation of the relevance of the reactions can also be difficult, perhaps requiring repeated open application testing, work-site visits etc. Monitoring of trends in patch test positivity can be effective in primary prevention of type IV allergy.