Williams syndrome (WS), caused by hemizygous deletion of 1.55–1.8 Mb of chromosome 7q11.23, has a recognizable behavior phenotype that is an important diagnostic sign. Individuals with WS are overly friendly, gregarious, empathetic, and loquacious, but have difficulty interpreting social cues and in making and keeping friends. The neurodevelopmental profile is characterized by overall intellectual disability, strength in concrete language, weakness in visuospatial construction, difficulties with sensory modulation, balance, and tool use, and problems with attention and anxiety. Structural and functional MRI studies demonstrate that gray matter deficiency in the intraparietal sulcus alters processing of spatial information in the dorsal stream (spatial) visual pathway, likely contributing to the visuospatial construction disability. Deficient regulation of the amygdala by the oribitofrontal cortex appears to underlie both the social disinhibition and the specific phobia common in WS. Continued study of cognition, behavior, neuroanatomy, and function in WS will continue to elucidate the neurogenetic underpinnings of human behavior. © 2010 Wiley-Liss, Inc.