Many different methods exist by which the radiosensitivity of cells, and hence of normal tissues and tumours, can be manipulated. Several of these can be traced to a common redox pathway involving competition between oxidising and reducing species. Others involve the biochemical enzyme systems required for removal of DNA lesions. At the present time most of these approaches are still actively undergoing basic research studies and have not found full application in cancer clinics. For any such approach to be of therapeutic use there must be a rationale for a differential effectiveness in tumours and normal tissues.