Down syndrome is the most common chromosome abnormality of man. The isolated occurrence of any one of the most of the protean systemic and ocular features of Down syndrome is not specific to the disorder. The associated occurrence of several of these features, however, has distinguished affected individuals as having a distinct entity for nearly 125 years. Recent advances in prenatal diagnosis have allowed the earlier detection, in utero, of chromosomal abnormalities. Although predisposing genetic and environmental influences remain for the most part unknown, advances in molecular biology are leading to a greater understanding of other common disorders that occur with an increased incidence in individuals with Down syndrome; these include Alzheimer's disease, acute childhood leukemia, congenital heart malformations, and immunologic abnormalities. Associated ocular disorders can significantly affect the quality of life of individuals with Down syndrome. As more children with Down syndrome live into adulthood, the ophthalmologist will play an increasing role in allowing them to lead productive and meaningful lives.