Retroviral vectors promote the efficient transfer of genes into a variety of cell types from many animal species. An important contribution to their utility was the development of retrovirus packaging cells, which allow the production of retroviral vectors in the absence of replication-competent virus. Because of their ability to transfer genes efficiently into cells that are difficult to transfect by other methods, retroviral vectors are prime candidates for gene transfer into human somatic cells. Indeed, a retroviral vector recently has been used to mark tumor infiltrating lymphocytes in patients with melanoma to follow the persistence and distribution of these cells following infusion into patients. Hopefully these vectors will soon be used for the treatment of disease by transfer of functional genes, or gene therapy. Here I will review the available packaging cell lines and their properties with a focus on their ultimate application to human gene therapy.