Abstract: The present invention is drawn to apparatus for transcutaneous photodynamic therapy (“PDT”) of a target tissue or compositions in a mammalian subject, which includes a light source that is external to the subject and is selected from among one or a plurality of laser diodes; light emitting diodes; electroluminescent light source; incandescent light sources; cold cathode fluorescent light sources; organic polymer light sources; or inorganic light sources, where the light source is adapted to direct the light in a direction lengthwise and parallel to a vessel wall comprising the lesion.