Laser-based spectroscopic techniques were developed for tumor tissue characterization utilizing different tumor-localizing substances. In particular, sensitization with the heme precursor delta-amino levulinic acid (ALA) administered topically, orally or intravenously was used for the induction of protoporphyrin IX (PpIX). The autofluorescence as well as the PpIX-related fluorescence signals were monitored, and tumor demarcation functions were calculated for different human malignant tumors, such as tumors in the urinary bladder and the prostatic gland, in the head and neck region, in the breast and in the gastrointestinal tract. In the gastrointestinal tract, colon tumors were examined as well as tumors and dysplastic lesions in the esophagus, where patients with Barrett's esophagus were examined. Time-integrated laser-induced fluorescence measurements utilizing a point monitoring fluorosensor and a multicolor fluorescence imaging system were performed in vivo in patients in different clinical specialities.