As phototoxic and photoallergic reactions have been recognised as unwanted skin manifestations caused by any of several hundred substances, drugs and chemicals, it is essential to determine the potential photo-sensitising properties of such substances before they are introduced in clinical therapy or made available on the market, in order to avoid such reactions. In cases of phototoxic reactions, the patient presents with skin changes resembling sunburn, sometimes accompanied by blistering, whereas in cases of photoallergic reactions the skin changes are similar to those of allergic contact dermatitis. The two most important aids to clinical investigation are determination of the erythema treshold, or the minimal erythema dose, and photopatch testing. The article reviews the basic mechanisms of photosensitisation, outlining the most important differences between phototoxic and photoallergic reactions, summarises the most frequent photosensitisers, and presents the diagnostic procedures, including the tests used in experimental phototoxicity.