This chapter reviews recent evidence describing the genetic underpinnings of several developmental disorders. It discusses the Down syndrome, focusing on the disorder of gene dosage, the Rett's disorder with emphasis on the persistence of genetic heterogeneity, and the Prader-Willi and Angelman syndromes, which involve a study in genetic imprinting. The chapter also discusses the Smith-Magenis syndrome (SMS) and Fragile X syndrome (FXS), with an attempt to understand the molecular basis of a developmental disorder. The Down syndrome is a phenotypically variable, relatively common cause of developmental disability. Overall Prader-Willi and Angelman syndromes represent developmental disorders whose genetic underpinnings are better understood than autism. Overlap between SMS and other developmental disorders, will allow for a better future understanding of the link between specific gene dysregulation and phenotypic expression. Increased understanding of glutamatergic neurotransmission may hold promise for future study of the neurobiology and treatment of autism spectrum disorder (ASD).