The diagnosis of fetal alcohol syndrome was initially restrictive in that only classic, full-blown cases were recognized by clinicians. Only 25% to 45% of mothers with chronic alcoholism have children with fetal alcohol syndrome. The diagnosis has since been expanded to include the milder forms, and thus the creation of terms such as "fetal alcohol effects," "alcohol-related birth defects," or "alcohol-related neurodevelopmental disorder." This conundrum of classification has defied resolution. Geneticists and dysmorphologists tend to reserve the diagnosis of fetal alcohol syndrome to children of alcoholic mothers with a recognizable physical phenotype. However, over time, psychologists, psychiatrists, and developmental pediatricians have come to recognize a characteristic behavioral phenotype. The width and breadth of the clinical spectrum are understandable in view of the expected variability imposed by teratogens, based on dose, time, duration of exposure, and genetic susceptibility. A chapter on animal research studies discusses comparable anatomical features and functional disability in man and mouse.