The many different oncological concepts of gene therapy can be classified into three major groups. These include immunopotentiation, compensation of oncogene and tumor suppressor gene alterations, and suicide gene strategies. Gene marker studies and the transfer of drug resistance genes to normal cells have also shown to be of some importance in oncology. Although there is no clear demonstration of the efficacy of gene therapy so far, it will most likely become part of the therapeutic armamentarium of oncologists within the next two to three decades.