The role of photodynamic therapy (PDT) in the treatment of in situ neoplasias and tumors of the skin is steadily increasing. Its principles of photodynamic action include an intratumoral enriched photosensitizer and light activation. Aminolevulinic acid (ALA) has demonstrated highest efficacy in topical PDT, and has become the most clinically useful. For actinic (solar) keratoses, topical ALA-PDT using Levulan Kerastick (20% topical solution, DUSA Pharmaceuticals) is already postulated to be the treatment of choice. In December 1999, the US FDA approved this topical product for the treatment of actinic keratoses. Levulan is well tolerated and leads to excellent cosmetic results with only minor side effects.