Gene therapy is based on the transfer and the expression of therapeutic genes in specific target cells. For the treatment of genetic diseases gene therapeutic approaches aim at replacement of the deficient gene or at the correction of the genetic defect. Malignant diseases and an increasing number of other acquired diseases are additional targets for gene therapeutic strategies. For gene therapy to become a potential future treatment option, safe and therapeutically efficient gene transfer into specific target cells is a central requirement. A variety of nonviral and viral vector systems have been developed. Nonviral vectors transfer genes are far less efficient than viral vectors, but they have advantages due to their low immunogenicity and their large size capacity for therapeutic DNA. To improve the function of nonviral vectors, the addition of viral functions such as receptor mediated uptake, enhanced endosomal release and nuclear translocation of DNA may finally lead to the development of an artificial virus. In contrast, natural viruses are already highly developed structures for the transfer of nucleic acids. Recombinant viruses can be used for efficient gene transfer. Retroviruses, adeno-associated viruses and lentiviruses are suitable for gene therapeutic approaches which are based on the permanent expression of therapeutic genes such as the correction of enzyme deficiencies or the manipulation of hematopoetic stem cells, as they are able to integrate nucleic acids into the cellular genome. In contrast, adenoviral vectors result in highly efficient, but transient gene expression and are therefore especially useful for the treatment of malignant tumors. Novel developments of viral vectors mainly aim at the reduction of immunogenicity, increase of capacity for therapeutic genes and at improved vector production. Viruses which replicate selectively in tumor cells leading to tumor cell lysis represent a novel generation of viral vectors, which can further be improved by the addition of therapeutic genes resulting in enhanced tumor toxicity.