Immunologic principles and methods for investigating dermal hypersensitivity were reviewed. The epidemiological aspects of chemically allergic contact dermatitis were summarized. Estimates by various governmental agencies and groups have indicated that skin problems account for up to 28% of the reported illnesses. NIOSH considers dermatological conditions to be one of the ten leading occupational diseases. Allergic contact dermatitis has been estimated to account for 20 to 25% of all cases of occupational dermatitis. The five most common causes of allergic contact dermatitis in the United States have been reported to be plant resins, nickel (7440020), p-phenylenediamine (106503), rubber chemicals, and ethylenediamine (107153). The skin has been characterized as a secondary immune organ in terms of innate resistance to infections and acquired specific immunity to foreign antigens. Langerhans cells are thought to be an important component of the skin immune system as they process antigens in the skin that induce humoral immunoglobulin-A, cutaneous cell mediated immunity, or allergic contact dermatitis. The immunological functions of the skin as they pertain to disease resistance and immunopathology were considered. Individual variations in the expression of allergic contact dermatitis were described. The immunologic basis of dermal hypersensitivity was considered. The mechanisms of allergic contact dermatitis and atopic dermatitis were discussed. The effects of immune system dysfunction induced by biomolecules, drugs, chemicals, and immunomodulatory physical agents and diseases on dermal hypersensitivity were considered. Current methods for assessing dermal hypersensitivity were reviewed. In-vitro screening methods for assessing dermal hypersensitivity were summarized.