PDT represents another modality for the treatment of human malignancy. Photoactivated hematoporphyrins have definite antitumor activity in both in vitro and in vivo experimental systems. Much of the early clinical work involved treatment of patients with advanced, recurrent disease who had not responded to conventional therapy. Because good responses with acceptable toxicity have been obtained in these patients, active investigation continued and is aimed at defining the most appropriate sites and applications for the technique. Because of the limited depth of light penetration in tissue, the most promising sites may be those where there is limited thickness of tumor, such as in superficial skin lesions or carcinomas in situ involving the aerodigestive tract, bronchial tree, or genitourinary tract. Other potential uses include those where PDT could be combined with surgical or chemotherapeutic debulking, such as pleural mesothelioma or advanced stage ovarian cancer. Whether PDT can be of benefit in surgical cases where the margins of resection are close is an interesting but speculative notion at the present time. Clinical trials with hematoporphyrin derivative PDT in the sites mentioned are in progress. Laboratory work to better understand HpD also continues, as well as investigations into alternative photosensitizers with improved tumor localization, less cutaneous photosensitivity, and absorption peaks at deeper penetrating wavelengths of light. Attempts at measuring singlet oxygen, if successful, will permit the development of more meaningful dosimetry in order to correlate response with actual tissue levels of the purported cytotoxic agent. Hopefully, these and other developments in the field of PDT will improve the treatment for patients with cancer.