Phototoxicity occurs when a substance is toxic only under the action of light, and can, depending on the mechanism of toxic action, be divided into type I and type II phototoxicity. Other types are sometimes recongnized, and sometimes toxicity occurs through more than one mechanism. Plants often use phototoxins for their defense. They may be harmful to man, but also exploited for medical treatments. On the other hand, drugs selected for other properties may show unwanted phototoxicity. Some fungi produce phototoxins. Due to other kinds of poisoning or to disease even the human or animal body may produce phototoxins and be harmed by them. Polycyclic aromatic hydrocarbons (PAHs), widespread environmental contaminants, have a potential to become toxic or acquire increased toxicity when they inter-act with natural or simulated sunlight. Because of their chemical structure numerous PAHs absorb energy in the UV waveband. Phototoxicity of PAHs occurs mainly via photosensitiza-tion and/or photomodification reactions: photosensitization reactions of bioaccumulated PAHs in biological matrices are regarded as important mechanisms for phototoxicity, but PAHs may also be photomodified into more toxic form e.g. via photooxidation. Phototoxicity of PAHs has been demonstrated in a variety of aquatic organisms, responses ranging from acute lethality to chronic effects. However, a variety of factors affecting the exposure of organisms to PAHs and to UV radiation, as well as interactions between multiple environ-mental factors and stressors present in natural conditions complicate the risk assessment for phototoxicity.