Certain dyes can be absorbed by biological tissue. If the dye absorbs light there, it can damage the tissue beyond repair. This effect is currently widely used to treat certain skin diseases and will undoubtedly be extended to internal diseases. A limited number of experiments have also shown its effectiveness against cancer in humans. Other dyes in tissue do no damage but merely fluoresce. The characteristics of this fluorescence may, in the future, be used to indicate the site and nature of disease in the body. Current optical technology seems particularly well adapted for use in these emerging areas of medicine. This paper suggests how optical technology, chemistry and clinical medicine may be combined in the photodynamic treatment of disease and forecasts the technological growth rates of the relevant biomedical specialties.