Historically, retroviral vectors have been the most frequently used type of gene delivery vectors for clinical gene therapy. In particular, vectors based on murine leukemia virus (MLV) and related retroviruses have been employed in almost half of the current gene therapy clinical protocols (http://www.wiley.co.uk/genetherapy). The major advantages of retroviral vectors are (1) ease of manipulation for insertion of the therapeutic gene; (2) ability to stably integrate into the target cell genome; (3) relatively high titer of the recombinant retroviruses; (4) a wide range of target species and cells that can be infected without any apparent adverse pathology; and (5) relatively simple procedure for preparation of the recombinant virus. However, the current retroviral vector have potential disadvantages as well, such as (1) requirement for cell division for integration, limiting their in vivo applications; and (2) random integration into host chromosome, resulting in possible insertional mutagenesis or oncogene activation. However, recent developments in virus packaging systems, use of modified or different envelope proteins for packaging, and modifications with the cis-acting regulatory elements to regulate transgene expression have allowed for the safe and efficient clinical application of retroviral vectors. This chapter provides a background on retrovirus-based vector systems as well as provides an update regarding improvements in retroviral vector for gene transfer.