The study of genetic disorders that affect neurodevelopment has led to a rich body of interdisciplinary research in genetics, neuroscience, and psychology. These collaborations have not only promoted a better understanding of genetic disorders themselves, but have also resulted in new discoveries about the connections between genes, brain, and cognition. When people consider genetic disorders that affect cognitive development they often think about single gene disorders such as fragile X syndrome or chromosome disorders such as Down syndrome. However, there is a growing recognition that many neurodevelopmental disorders have strong genetic components even though their genetic underpinnings may be less well understood than those diagnosed through genetic testing. And, increasingly, cross-disorder comparisons with overlapping phenotypic variability are proving to be useful models for understanding the interplay of genes, brain, and behavior across development. Given the increasing recognition of the role that genes play in developmental disorders, an exhaustive survey of disorders that affect cognitive development is beyond the scope of any one book. The purpose of this book is to represent some of the ways in which a number of disorders, both those diagnosed through genetic testing, and those identified through their physical and behavioral phenotypes, are being used to test models of neurobehavioral development and to understand relations between genes, brain, and behavior.