Photodynamic therapy is a new endoscopic procedure for the treatment of malignant or premalignant structure lesions. The underlying principle is selective destruction of tumor cells, which, after accumulating a so called photosensitizer, are irradiated with light of a particular wavelength. Clinical experience to date suggests that, in the field of gastroenterology, endoscopically accessible tumors located within the esophagus, the stomach, or the colorectum of patients with a high surgical risk are particularly suitable for such treatment. If such tumors are limited in size, they can often be treated with curative intent, and remission rates better than 90% can be achieved. Also, palliatively, the tumor mass can be reduced appreciably in a large percentage of patients. The main side effect of this treatment is photosensitization of the skin; perforations and hemorrhage are rare. Improvements in technology and photosensitizers suggest that the procedure will become an established part of the therapeutic concept for gastrointestinal diseases.