Gene therapy is used to prevent or treat genetic diseases. To treat genetic diseases, transfer of genes that correct the effects of the mutation that causes the disease to the patient. In gene therapy, the therapeutic gene is transferred to target cells through a vector and viral vectors are most used. Retroviruses (lentiviruses), adenoviruses, adeno-associated viruses, and herpes virus vectors are common as viral vectors. In this section, biological methods used especially in gene therapy stages examined.