INTRODUCTION, Photodynamic therapy (PDT) utilizes a photosensitizer and light to generate reactive oxygen species (ROS) which cause tissue damage. PDT involves a photochemical method of injury which is different from the heat-induced effects typically observed during many light-tissue interactions.PDT has been utilized to treat a wide range of benign, premalignant, and malignant dermatologic disorders. Common uses such as treatment of actinic keratoses and skin cancers are discussed in other chapters. PDT has also been evaluated for treatment of other cutaneous disorders, including cutaneous T-cell lymphoma (CTCL), psoriasis, and port wine stain (PWS) birthmarks (Fig. 10.1). PDT photosensitizers can be applied topically or delivered systemically ( oral or intravenous). Because of ease of use and fewer potential side effects, dermatologic PDT most commonly utilizes topical photosensitizers. In the USA, the only FDA-approved topical photosensitizer is 5-aminolevulinic acid (ALA) (see Fig. 1.IA). During clinical use, nonfluorescent, nonphotodynamically active ALA is applied to the skin where it is transformed into highly fluorescent and photodynamically active protoporphyrin IX (PpIX) via the heme cycle (see Box 1.2). However, in some cases, systemic photosensitizer delivery may offer significant benefits, e.g. in treatment of PWS where such administration offers an opportunity for vascular compartmentalization and selective vascular destruction.