Photodynamic therapy (PDT) involves selective photosensitization of a target tissue by means of a topically or systemically administered agent which is then activated by light to effect an oxygen dependent cytotoxic reaction. The production of reactive oxygen intermediates, including singlet oxygen, is localized to where the photosensitizer accumulates and induces apoptosis and vascular endothelial damage. First-generation photosensitizers are haematoporphyrin derivatives and are effective in treating certain nonmelanoma skin cancers. However, they induce cutaneous photosensitization for at least 4-6 weeks and have a limited role in dermatology. Many second-generation photosensitizers, such as 5-aminolaevulinic acid, are associated with less prolonged photosensitization. Although PDT remains largely experimental, it has potential applications in both benign and malignant skin disease.