Somatic gene therapy involves the introduction of novel genetic material into somatic cells to express therapeutic gene products. This emerging technology holds great promise for the treatment of both inherited and acquired diseases. This review summarizes the principles of gene therapy and approaches that are being investigated in experimental animals and clinical trials. These include the construction of recombinant viruses capable of carrying genes into cells by the process of infection as well as the use of DNA molecules that are capable of being used like conventional medicines. Some methods for gene therapy lead to permanent insertion of genes into targeted cells, while others are designed to express a therapeutic product with a defined half-life and duration of action. The goal is to establish site-specific and regulated expression of therapeutic products. The demonstrated safety and public acceptance of initial clinical trials will lead to widespread investigation of applications in both medicine and surgery in the near future.