RNA viruses including retroviruses, alphaviruses, flaviviruses, rhabdoviruses, measles viruses, paramyxoviruses, and picornaviruses have contributed substantially to gene therapy. A large number of disease indications have been targeted in both preclinical studies and clinical trials. RNA viruses have been subjected to cancer immunotherapy resulting in therapeutic efficacy in animal models and in cancer patients. Lentivirus-based Factor VIII expression provided therapeutic efficacy in a hemophilia A mouse model. Moreover, lentivirus vectors have demonstrated promising results in clinical trials in patients with sickle cell anemia. Neurological disorders such as Parkinson’s disease and Alzheimer’s disease have been subjected to lentivirus-based gene therapy showing improved motor function and cognitive activity, respectively. Furthermore, cardiovascular, metabolic, ophthalmologic, and lung diseases have been targeted. Long-lasting cure in patients with X-linked severe combined immune deficiency has been achieved with retrovirus vectors and lately also with lentivirus vectors.