Mental retardation or intellectual disability affects about 3% of the general population. One half of it is attributed to genetic factors and is therefore inheritable. Genetic basis of mental retardation is poorly understood. However information revealed from the human genome project has enabled to analyze the genetic determinants that have causative role in the onset of the disease. This book provides a compendium of basic information on the genetic factors that lead to this disability, methodology to screen the causative genetic defects and identification of new causative determinates. This information is critically important in the diagnosis and prognosis of mental retardation at an early stage, resulting in better management of the disease, ultimately leading to improvement in life of the affected individuals. The book offers a valuable source of information for the graduate and post graduate students in genetics, biology and medicine.