Down syndrome, which is the result of the trisomy of Chromosome 21, is the most common genetically defined cause of intellectual disabilities. The estimated number of people with Down syndrome in the United States is approximately 300,000, and this figure is expected to continue increasing due to projected increases in the life expectancy of people with Down syndrome. Although this population trend reflects improvements in the general health care of individuals with Down syndrome, there has not been a parallel progress in the understanding of the pathogenesis and potential treatment of the psychological and neurological components of this genetic condition. These include various degrees of intellectual disability, increased incidence of seizure disorder in relation to the general population, motor dysfunction (including hypotonia), abnormal oculomotor and vestibular functions, substantial visual deficits, a neuropathology indistinguishable from Alzheimer disease, and increased incidence of major depression and dementia in adults.