Chronic actinic dermatitis, synonymous with the photosensitivity dermatitis and actinic reticuloid syndrome, presents as a dermatitis and/or a pseudolymphomatous eruption. Abnormal photosensitivity to ultraviolet (UV) and often visible radiation is a feature. Many patients also have multiple contact allergens. Histopathologic features vary, with a spectrum from mild dermatitis to pseudolymphomatous (reticuloid) features. The essential tests to make the diagnosis and to guide advice on avoidance of the responsible wavelengths and any contact allergens are phototesting and patch testing. Chronic actinic dermatitis can be regarded as a disorder of increased susceptibility, for reasons that remain uncertain, to develop delayed-type allergic responses to both endogenous photoallergens and exogenous allergens. Treatment consists of detailed advice on sunlight and allergen avoidance (guided by the results of investigations), topical corticosteroids, and emollients. When these measures are insufficient alone, systemic immunosuppressives may be considered: systemic prednisolone for acute exacerbations or azathioprine if systemic treatment is required for more than a few weeks.