The therapeutic concept of photodynamic therapy (PDT) with topically applied δ-aminolevulinic acid (ALA) relates to endogenously formed porphyrins which are acting as photosensitizers. Topical application of ALA induces porphyrin biosynthesis preferentially in tumors. Irradiation of porphyrinenriched tumor tissue induces necrosis by formation of reactive oxygen species. Intratumoral accumulated porphyrins can be visualized by exciting their fluorescence under Wood’s light irradiation (=UVA+ visible light), which is used for fluorescence diagnosis with ALA-induced porphyrins (FDAP). In the present studies ALA-induced porphyrin synthesis was measured in cells, in organ culture systems and in vivo. The efficacy of FDAP and PDT was evaluated for several cutaneous diseases.