Photodynamic therapy (PDT) is a minimally invasive treatment for malignant and nonmalignant diseases that uses light-activated photosensitizers to destroy or modify cells or tissues. It has been approved by health agencies in several countries for cancers of different sites and stages, and for treatment of age-related macular degeneration, various benign skin conditions and localized bacterial infections. PDT research has led to significant advances in the technologies (both light and drug) and to understanding of the basic mechanisms of action, as well as extending the range of clinical applications. The themes covered in the papers of this special issue include newly developed photosensitizers (PS) and delivery systems, optical technologies for monitoring PDT dose, and preclinical and clinical applications, together with new fundamental studies of phototoxicity.