Abstract
Photodynamic therapy is a method for local destruction of tissue or organisms by generating toxic oxygen and other reactive species using light absorbed by an administered or an endogenously generated photosensitiser. It is a highly promising treatment for patients with cancer. More recently it has found increasing use as a method of therapy for non-cancerous illnesses. It depends on the exploitation of natural and vital reactions widespread in nature that have driven and preserved life on this planet. Following administration of a photosensitiser or its precursor there is an accumulation or retention in areas of cancer and disease relative to adjacent normal tissue. The photosensitiser is inactive until irradiated by light, following which cellular destruction occurs. The clear attraction of this method is the possibility of some targeting of the disease by drug and by the area irradiated. This explanation although oversimplified has been the reason for the scientific and clinical interest in photodynamic therapy. An understanding of evolutionary photobiology is enormously helpful to understand disease response and clinical outcomes.