Tumor-suppressor genes play pivotal roles in maintaining genome integrity and in regulating cell proliferation, differentiation, and apoptosis. Their loss-of-function mutations are related directly to tumorigenesis. Thus, use of tumor-suppressor genes as anticancer therapeutics has been investigated rigorously in both experimental and clinical researches. Transfer of various tumor-suppressor genes directly to cancer cells has been demonstrated to suppress tumor growth via induction of apoptosis and cell-cycle arrest and, in some cases, with evidence for bystander effects. Various studies also have shown that combination of tumor-suppressor gene therapy with conventional anticancer therapy can yield synergistic therapeutic benefits. Clinical trials with tumor-suppressor genes, especially the p53 gene, have demonstrated that the treatment is well tolerated, and; favorable clinical responses, including a pathologically complete responses, have been observed in a subset of patients with advanced disease or with cancers resistant to conventional therapy. Yet, current gene replacement approaches in cancer gene therapy must be improved if they are to have a broader clinical impact. Efficient systemic gene delivery systems will be required ultimately for treatment of metastatic disease. In this review, we have recently summarized achievements in tumor-suppressor gene therapy with a focus on the p53 gene.