Gene therapy has emerged as a new concept of therapeutic strategies to treat diseases which do not respond to the conventional therapies. The principle of gene therapy is to Introduce genetic materials into patient cells to produce therapeutic proteins in these cells. Gene therapy is now at the stage where a number of clinical trials have been carried out to patients with gene-deficiency disease or cancer. Genetic materials for gene therapy are generally composed of gene expression system and gene delivery system. For the clinical application of gene therapy in a way which conventional drugs are used, researches have been focused on the design of gene delivery system which can offer high transfection efficiency with minimal toxicity. Currently, viral delivery systems generally provide higher transfection efficiency compared with non-viral delivery systems while non-viral delivery systems are less toxic, less immunogenic and manufacturable in large scale compared with viral systems. Recently, novel strategies towards the design of new non-viral delivery system, combination of viral and non-viral delivery systems and targeted delivery system have been extensively studied. The continued effort in this area will lead us to develop gene medicine as "gene as a drug" in the near future.