Numerous anomalies involving the sex chromosomes (X or Y) have been documented and their impact on development, learning, and behavior studied. This article reviews three of these disorders, Turner syndrome, Klinefelter syndrome, and Lesch-Nyhan disease. Each of these three is associated with one or more selective impairments or behavioral abnormalities. The origin of these disorder-specific impairments is becoming progressively understood at the level of genetics, central nervous system structures and functions, and neurotransmitter action. Accordingly, knowledge of the three disorders is important not only to help practitioners provide services for affected students but also to enhance their general understanding of the physiological influences on behavior. (Contains 1 table.)