Genetic disorders in children can have highly variable effects. Even relatively common disorders may go undiagnosed and untreated by clinicians who are not familiar with the range of atypical cognitive or behavioral symptoms possible in an affected child. Recent research in genetics and brain development has altered the phenotypic description of various disorders, but this new knowledge is not readily available to practitioners. This collection provides a single resource that will help clinicians, pediatricians, neuropsychologists, educators, and others use the latest research to identify and treat a variety of genetic disorders as early as possible.The chapter authors report on the full range of phenotypes, including subtle or atypical variants, for each disorder. They describe disorders that have wide-ranging cognitive phenotypes and a well-understood genetic etiology (including Fragile X, Turner, and Klinefelter syndromes), discussing the genotype that leads to the syndrome, the medical implications, and the behavioral or psychological consequences. The chapter authors also report on more complex categories of etiologies, including congenital hypothyroidism and metabolic disorders, the genetic components of which are not completely understood. Finally, they go beyond diagnosis, discussing genetic counseling, family adaptation, and early intervention options for the preschool- and school-age years.