In this article are reviewed the various hypotheses concerning the etiology of the persistent light reaction and photosensitivity dermatitis, which occurs among older people and is characterized by an extreme photosensitivity that sometimes evolves into a pseudolymphoma, that is, actinic reticuloid. An etiologic agent can sometimes be demonstrated, but the precise pathogenetic mechanism is unknown. Clinical experience and experimental research indicate that the cause is most probably multifactoral: contact allergenic, photoallergenic, phototoxic, immunologic, and metabolic factors are involved, but their precise roles in the origin of the extreme photosensitivity are still unclear. Localized persistent light reactivity could result from the continued effects of the original photoallergen, but generalized photosensitivity is more difficult to account for. Among the many different hypotheses, two are of special interest: autosensitization of skin proteins with endogenous photosensitizers and cellular hypersensitivity to light, as is manifested by fibroblast cultures of actinic reticuloid patients.