Contact dermatitis, in all of its protean manifestations, is one of the commonest disorders that affects the skin. The clinical picture may be clearly evident, yet a history to support the diagnosis is sometimes difficult to elicit. It also happens that the agent suspected will not infrequently fail to produce a reaction when applied to the skin in appropriate dilution, in the usual manner. We are forced, in such a situation, to assume that the history is inadequate or inaccurate, that the disorder is due only to sensitivity in the areas involved, or that some unknown factor other than mere contact is necessary to activate the process. In the discussion which follows, I wish to present evidence which indicates that light is an essential factor in the development of certain dermatoses produced by local application of selected sensitizing agents. Contact photodermatitis is the result of an