AbstractPhotodynamic therapy (PDT) requires a photosensitizer within a target tissue and activation by proper‐wavelength radiation at appropriate energy level to achieve complete eradication of that tissue. Whereas the first clinical results were reported nearly a decade ago, only in the past few years has the necessary technology been available for development of phase III controlled clinical trials. Photofrin II, the trade name for a mixture of oligomeric porphyrins, is effective in treatment of a wide variety of malignant tumors and is the subject of Phase III controlled trials in bladder and lung. New photosensitizers are being developed that may be both more efficient and less likely to cause the generalized cutaneous photosensitivity associated with Photofrin II. Further advances in light sources and delivery systems portend an even broader acceptance of PDT in medical and biological scienc