Retrovirus vectors are extremely versatile tools that have been used in a wide range of systems. They have recently been employed in studies of developmental neurobiology and show great promise for future work in the neurosciences [see reviews by Price et al. (1987) 71 and Cepko (1988a,b, 1989) 12 13 14 ]. Although several features contribute to the popularity and versatility of retroviruses, their most important attribute as vectors is their efficient integration system. This feature, which is generally unlimited in host range, has enabled the transfer of genes isolated from a wide range of host species into many different types of cells. Gene transfer can be accomplished in vitro or in vivo. The in vivo capacity offers a major advantage over other gene transfer techniques, such as transfection or electroporation, which are generally limited to in vitro use. A discussion of the basic virology that underlies vector functions, as well as potential applications and protocols follows.