Significant advances have been made in the last 20 years in understanding the basic biology of the normal nervous system and in elucidating molecular and cellular mechanisms underlying neurological disease. This progress has generated, for the first time, a realistic possibility of treating what have historically been common and tragically untreatable diseases of the nervous system. In particular, therapeutic delivery of genes to the degenerating, injured or developmentally-deficient nervous system offers the potential to prevent cell death, induce new growth and restore function. Clinical trials of gene therapy are beginning to move forward in several neurological disorders. We have thereby begun the transition to molecular-based medicine which has the potential to alter the landscape and prognosis of neurological disease.