Cutaneous photosensitivity diseases may be idiopathic, produced by endogenous photosensitizers, or associated with exogenous photosensitizers. Those caused by exogenous agents include phototoxicity, photoallergy, and the exacerbation or induction of systemic disorders in which photosensitivity is a prominent clinical manifestation. Phototoxic disorders have a high incidence, whereas photoallergic reactions are much less frequent. The action spectra for most phototoxins and photoallergens lie in the UVA range. Phototoxic and photoallergic reactions can be distinguished on the basis of pathogenesis, clinical characteristics, diagnosis, and management. Drugs capable of causing phototoxic reactions include psoralens, porphyrins, coal tar, antibiotics, and nonsteroidal antiinflammatory agents. Drugs capable of causing photoallergic reactions include topical antimicrobial agents, fragrances, sunscreens, nonsteroidal antiinflammatory agents, plants, and psychiatric medications. Drug-induced systemic diseases in which photosensitivity is a prominent component include drug-induced lupus erythematosus, porphyria, and pellagra.