Gene transfer into the cells of the nervous system has the potential to provide interesting information for many fields of the neurosciences, including neurochemistry, neurophysiology, developmental neurobiology, molecular neurobiology and neuropathology, and perhaps may even to contribute to the ongoing debate between psychologists and neurobiologists about the nature of learning and memory. Although the field is in its infancy, however, there are already several different approaches for delivering DNA into the cells of the nervous system. These include transfection of cells with DNA, microinjection of fertilized eggs to construct transgenic animals, and infection with viral vectors. Each of these systems has its advantages and drawbacks, and improvements in the technology are needed. Many different genes of interest to neurobiologists have been cloned, including those for enzymes that synthesize or degrade neurotransmitters, neuropeptides, neurotransmitter receptors, ion channels, and proteins that mediate second messenger effects. Functional characterization of these genes using gene transfer technology has already yielded information. At present far more experiments have been done in tissue culture systems than in vivo. In five years time it should be possible to write a comprehensive review, detailing the technological improvements to allow routine gene transfer in vivo, the cloning of many more genes of interest to neurobiologists, and functional studies of these genes on a molecular level that demonstrate unifying and diversifying principles of neuronal function.