The evolution of Photodynamic Therapy (PDT) to a fully developed treatment modality requires the development of appropriate dosimetry to ensure proper quality control during treatments. The parameters measured for PDT quality control were the drug accumulation and the optical penetration depth. These methods were tested in vitro in photochemical assays and in tissue simulating phantoms. Pilot clinical trials were conducted and in vivo measurements were perform in patients undergoing endoscopic screening for esophageal diseases and photodynamic therapy of esophagus, lung, oral cavity and skin.