Current approaches to the treatment of cancer (radiation and chemotherapy) are limited by the toxicity to normal tissues and the sensitivity of tumor cells to these treatments. The use of gene delivery vector systems for the introduction of genetic elements into normal and neoplastic tissues which code for proteins that protect the normal chemotherapy-sensitive tissues and sensitize the tumor cells to radiation and chemotherapy, may contribute to an overall improvement in the outcome of cancer treatment programs. This chapter will review the clinical trials either completed or about to be initiated which are designed to accomplish these goals.