Photodynamic therapy (PDT) is still an experimental modality using red light in conjunction with a systemic or topical photosensitizer. The photosensitizers have a longer retention time in malignant tumors compared with the normal surrounding tissue. Irradiation with a red light at wavelength of 630 nm results in photochemical generation of cytotoxic singulet oxygen. The systemic sensitizer mostly used is hematoporphyrin derivative (HPD) and the light sources are argon-pumped dye lasers, gold vapour lasers and even more simple light sources based on conventional lamps. The most important side effect of systemic application of HPD is light hypersensitivity in the UVA range, which lasts up to several weeks. Multiple studies in recent years have proved that superficial basal cell carcinomas, squamous cell carcinomas, Bowen's disease and epidemic Kaposi's sarcoma respond to systemic PDT. Up to now, however studies with greater numbers of patients and a sufficient follow-up have been lacking. Topical application of sensitizers like amino-levulinic acid (ALA) and tetraphenylporphine sulfonate (TPPS) have been used for the treatment of superficial basal cell carcinomas and squamous cell carcinomas. Possible future applications of PDT are virus papillomas and psoriasis. Even if PDT is an experimental therapeutical modality at present, it may become more clinical relevant in the future.