Mental retardation is a disability characterized by significant limitations both in intellectual functioning and in adaptive behavior as expressed in conceptual, social, and adaptive skills, which originates before the age of 18. The prevalence of mental retardation among elementary school children in Korea is estimated as 9∼12/1,000. In most cases of mental retardation, the interplay of genetic and environmental influences remains poorly understood. However, recent ad-vances in molecular genetic techniques have enabled us to understand more about the molecular basis of several genetic syndromes associated with mental retardation. This article presents a review of three common genetic conditions causing mental retardation-Down syndrome, fragile X syndrome, and Prader-Willi syndrome-with the summary of the effectiveness of early intervention for the children with mental retardation. (J Korean Acad Rehab Med 2006; 30: 103-110)