In several countries throughout the world the photosensitizer porfimer-sodium has been approved for systemic photodynamic therapy (PDT) for different oncological indications. However, owing to the prolonged photosensitization entailed, the use of this porphyrin derivative is restricted. Currently, the most promising sensitizers in dermatology that can be applied topically are 5-aminolevulinic acid (ALA) or ester derivatives that are precursors of heme biosynthesis. ALA has shown good clinical and excellent cosmetic results in superficial skin cancer and precancerous conditions, e.g. superficial basal cell carcinoma (BCC), or actinic keratoses (AK): phase III studies have demonstrated its efficacy especially in Bowen's disease and AK. ALA-PDT for AK was therefore approved by the FDA in late 1999, and the corresponding registration process is currently in train in Europe. Besides its usefulness in oncological therapy, ALA also has a unique feature that can be exploited for diagnostic purposes: after topical or systemic application protoporphyrin IX is induced rather selectively in epithelial tumors, with a high tumor-to-surrounding tissue ratio, which can be visualized after excitation with light. By using a CCD camera system together with digital imaging, the contrast of the acquired fluorescence images can be significantly enhanced and allows the determination of a threshold, which can be utilized either for a directed biopsy or for preoperative planning when Mohs' surgery is scheduled. At present, the routine employment of such systems is being assessed in prospective studies.