Radiation has been a well-established modality in cancer treatment for several decades. Significant improvements have been achieved in radiotherapy over the years due to technological advances and development of facilities for delivery of charged particles such as protons. Nonetheless, the potential for tumor control with radiotherapy must always be carefully balanced with the risk for normal tissue damage. In addition, tumor cells outside the immediate field of radiation exposure or that have metastasized to distant sites are not destroyed. Gene therapy offers many exciting possibilities by which the overall efficacy of radiotherapy may be improved, while minimizing unwanted side effects. This review highlights several of the most promising gene transfer approaches that are currently being evaluated in combination with radiation in the treatment of cancer. Results from studies utilizing genes encoding molecules that function in apoptosis, radiosensitization, immune up-regulation, angiogenesis, DNA repair, normal tissue protection from radiation damage, and tumor targeting are discussed. The evidence indicates that many of these innovative gene-based strategies have great potential to augment radiotherapy, as well as other established forms of cancer treatment, in the near future.