Tremendous advancements have seen in the past decade in the diagnosis of mental retardation. In present scenario prevalence of mental retardation is also increasing so more research has to be carried out to unearth the causes of mental retardation which will help in treatment and reduction in its prevalence. Mental retardation is the most common developmental disorder which is affecting almost 3% of the population. Mental retardation (MR) is a particular state of functioning that begins in childhood and is characterized by decreased intelligence and adaptive skills. The causes of mental retardation need not be present at birth. Mental retardation may be acquired during later development (but prior to eighteen years of age). Causes of mental retardation are numerous and include genetic and environmental factors. They are usually grouped into: (i) prenatal, (ii) perinatal (around the time of birth) and (iii) postnatal causes. In at least 30 to 50 percent of cases, physicians are unable to determine etiology. It is noted that diagnostic capability-clinical and laboratory-must be increased for 50% reduction in the prevalence of severe mental retardation to be achieved in the foreseeable future.