Cognitive disorders affect thinking and perceptual processes and the acquisition of knowledge and new information. They have an enormous societal impact because special educational resources are required, and independent living often cannot be achieved. Learning problems may lead to behavioral disorders in the home and community. The pathogenesis of most mild and moderate cognitive disorders is poorly understood. Severe cognitive impairment is usually accompanied by somatic abnormalities, and an etiology can be identified in many cases. Specific treatments are available for disorders such as congenital hypothyroidism, some metabolic acidurias, and congenital toxoplasmosis. Other disorders affecting cognition such as fetal alcohol syndrome, maternal cocaine and heroin exposure, HIV encephalopathy, and prematurity require aggressive prevention and education to reduce their occurrence. The recent advances in molecular genetics offer a faster and better method of diagnosing fragile X syndrome, now recognized as the most common inheritable cause of mental retardation. In the future, DNA analysis may elucidate the basis of many other cognitive disorders.