Several substances, e.g., hematoporphyrin derivatives (HpD), dihematoporphyrin ether/ester (DHE), phthalocyanines, porphycenes, and other drugs are known to be temporarily and selectively stored in tumors after systematic application. This transient marking opens up new perspectives for diagnostic and therapeutic procedures. The marker most commonly used today is DHE intravenously injected at doses of 0.2 up to 3.0 mg/kg bodyweight for diagnosis and therapy respectively. The corresponding clearance intervals after injection of DHE range from 3 - 48 h and 25 - 75 h. The highly sensitive two-wavelength laser excitation method with computerized fluorescence imaging offers great advantages for the detection of photosensitized tumors and adds support to conventional diagnostic techniques. Photoinduced production of singlet oxygen is said to be the initial process leading to tumor destruction. Homogeneous irradiation of the area to be treated and a reliable light dosimetry are prerequisites for an effective tumor therapy. Standard instruments for a routine application so far do not exist. Integral irradiation techniques and special laser fiber modifications, however, are under development, which guarantee a uniform distribution of light on the area to be treated. Positive results are such treatments--especially in urology, pneumology, and otorhinolaryngology--indicate the future potential of photodynamic therapy of tumors.