Recent advances in molecular biology have opened new avenues of basic genetic engineering technology and have made possible the application of this technology in clinical human gene therapy. Replication-defective viral vectors and biocompatible materials, eg, liposomes, have been developed as vehicles to introduce potentially therapeutic genes into mammalian cells. Over the past 2 years, this technology has increased the possibilities for therapy in numerous genetic diseases. Approaches at the molecular level have also demonstrated that one of the mechanisms of human cancer development is overexpression of dominant oncogenes, expression of mutant oncogenes, or specific chromosomal deletions or mutations that induce inactivation of tumor-suppressor activity. This concept suggests that the introduction of antisense oncogenes and wild-type tumor-suppressor genes, eg, p53, could halt or reverse these mechanisms, thus having a therapeutic effect in cancer. Moreover, evidence that the immune system is capable of eliminating tumor cells in numerous animal models has suggested gene therapy approaches for the delivery of cytokines, which promote the activation of cytotoxic immune responses against the malignant tissue. The efficacy of these gene therapy protocols is now being evaluated in both animal model systems and clinical trials. This article reviews recent highlights in this rapidly evolving field.