In 1973, researchers Kenneth Jones and David Smith at the University of Washington labeled a set of characteristics of children who had been exposed prenatally to alcohol as fetal alcohol syndrome (FAS). FAS characteristics include growth deficiency, facial morphological abnormalities, and/or central nervous system dysfunctions. Although overall rates of FAS might be underestimated as a result of difficulty in making a diagnosis during early development, current evidence suggests that the prevalence rates in the general population range from 3.3 to 9.7 cases per 100,000 live births. FAS is the leading known cause of mental retardation in the United States and perhaps in many other countries. This entry discusses the prevalence, cause, defining characteristics, and prevention of FAS.