Photosensitive skin reactions occur when human skin reacts to ultraviolet radiation or visible light abnormally. The forms of photosensitivity are phototoxicity and photoallergy. Phototoxic disorders have a high incidence, whereas photoallergic reactions are much less frequent in human population. Several hundred substances, chemicals, or drugs may invoke phototoxic and photoallergic reactions. In order to avoid photosensitive reactions it is essential to determine the photosensitizing properties of such substances before drugs are introduced in therapy or products made available on the market. The article reviews the mechanisms of photosensitization, explains the most important differences between phototoxic and photoallergic reactions, summarizes the most common photosensitizers, and presents the clinical features and diagnostic procedures of phototoxic and photoallergic reactions.