A contact dermatitis is more often due to skin irritation than to allergic reactions. About 80% of occupational dermatoses are explicable as toxic reactions of the skin, 20% as allergic reactions. Frequency and severity of the diseases depends on the nature and concentration of a substance as well as from the duration of its action on the skin. Defense mechanisms against chemicals are influenced by sebum content, humidity and the integrity of the horny layer. From a practical standpoint it is important to differentiate toxic from allergic reactions in a patch-test. It seems that allergic reactions are diagnosed too often. The determination of the critical non-toxic concentration requires the evaluation of a sufficient number of patients. The differentiation of irritation and allergy is tendered more difficult by individual factors. The therapy of toxic and allergic contact dermatitis is discussed.