Photodynamic therapy, PDT, is a method to diagnose and treat cancer. In PDT a sensitizer is administered to the patient and this sensitizer is accumulated in tumors. If the sensitizer-containing tumor is subjected to a laser of a specific wavelength the tumor is fluorescing allowing diagnostics. If other wavelengths are used a process involving reactive oxygen species and singlet oxygen is started and the tumor cells are killed. This process thus requires oxygen as well. This thesis investigates how UV-induced damage of the skin and different physiological factors of the skin influences the uptake of 5- aminolevulinic acid, ALA, and its conversion to the active sensitizer protoporphyrin IX, PpIX. It shows that UV-induced damage affects both the uptake and production of PpIX. UV-induced damage lowers the PpIX produced after ALA application both if the damage is acute and in chronically UV-affected skin. The PpIX production differs inter and intra individually. When looking how different physiological factors affect the PpIX production after topically applied ALA the thesis shows that an increase of temperature increases the production. No correlation between the formation of PpIX and the density of hair follicles was found and a weak correlation was seen comparing the epidermal and total dermal thickness and PpIX production The thesis also shows how PDT is used in treating laryngeal malignancies. It shows that it is possible to cure laryngeal tumors (both squamous cell carcinomas and sarcomas) using PDT primarily, and that the cure rate as well as outcome of voice and patient safety is comparable to the conventional treatment modalities. PDT can also be used as a function and organ sparing treatment for recurring laryngeal cancers, both squamous cell carcinomas and sarcomas.