Photodynamic Therapy is based upon light-activation of a substance which is selectively and pre-emptively concentrated in the target cells. The target cells are those which are tumoural, actinic, aged or inflammatory. Activation causes the death of these cells, which will later be substituted with healthy ones. It is the first-choice therapy in treatment of diffuse facial actinic keratosis, and causes production of new collagen. It results in evident facial photorejuvenation, and is used in treatment of active acne (only when traditional therapies have proved ineffective) and in treatment of some forms of superficial cutaneous tumours.