Allergic contact dermatitis is both an important clinical problem and a model system for lymphocyte-mediated pathologic changes. Elicitation of allergic contact dermatitis requires interaction of antigen with epidermal Langerhans cells, followed by migration of the Langerhans cells to the lymph nodes to present antigen to T lymphocytes. These activated T lymphocytes must then home to the antigen-exposed skin. Adhesion molecules such as LFA-1 and ICAM-1 have a role in this homing. Only a small proportion of the T lymphocytes in the skin lesion are specific for the inducing antigen. Studies of poison ivy (urushiol dermatitis) have determined this fraction to be less than one per 100 infiltrating lymphocytes. By a variety of amplification mechanisms, it is possible for this small number of antigen-specific T lymphocytes to induce the pathologic changes of allergic contact dermatitis. Improved understanding of this condition should result in increased knowledge of the pathogenesis of a variety of T lymphocyte-mediated skin conditions.