Gene therapy is a novel and promising treatment for inherited and acquired genetic disorders. Basic laboratory investigations and clinical trials have shown that gene therapy in vivo is feasible, results in biological responses, and is relevant to different types of human diseases, To achieve clinical efficacy, current research had focused on improving the efficiency and specificity of gene transfer. Furthermore, gene therapy unlocks new research approaches and offers important insights into disease mechanisms. The emphasis of gene therapy has also shifted from the treatment of inherited genetic diseases to cancer. Despite the significant progress made in our understanding of gene therapy, considering conceptual and practical limitations still exist. Evidence of the efficacy of gene therapy, however, is compelling. Many of the current problems of gene therapy should be overcome and gene therapy will become an important treatment modality for genetic diseases.