The developing central nervous system (CNS) crucially depends on the maintenance within embryonic cells of precise levels of retinoic acid (RA), a low-molecular-weight, lipophilic compound derived from vitamin A. The level of RA must not rise or decrease excessively during pregnancy, or CNS damage will result. Although the broad effects of excess or insufficient levels of RA on the nervous system have been known for some time, it has been only within the last 10 years, through the use of anatomically specific molecular markers, that we have begun to understand the specific role of retinoic acid in body patterning and CNS development.