Over the past decade, viral vectors have slowly gained mainstream acceptance in the neuroscience and genetics communities for the in vivo study of gene function [1]. Using stereotactic techniques, it is possible to characterize neuroanatomical relationships through the delivery of neurotropic viral vectors to specific brain regions. More sophisticated studies combine viral vectors with other methods of genetic manipulation such as germline transgenic mice. As more is learned about the properties of different viral vectors, it has become possible to use viral vectors to test hypotheses about the function of genes, through targeted in vivo delivery to the central nervous system (CNS). The effects of gene expression in the brain can be measured on the molecular, biochemical, electrophysiological, morphological, and behavioral levels. We propose that viral vectors should be considered as part of an integrated functional genomics platform in the CNS.