Mental retardation (MR) occurs in 2-3% of the general population. Prevalence of milder MR is seven to ten times more than severe MR. Cause of severe MR can be determined in 60-70% of cases, as compared to mild MR where 35-55% remain idiopathic. The diagnostic process is aided considerably if the timing of a developmental insult can be determined: prenatal, periatal, postnatal (not mutually exclusive). History plays a pivotal role in approaching a diagnosis. After clinical evaluation one should be able to assess whether the disorder is static or progressive; approximate developmental quotient; possible timing of insult and possible underlying genetic etiology. Investigations should be based on history and physical examination. The important category of tests include: thyroid function tests, cytogenetic studies, metabolic work-up, fragile-X screening, radiological investigations, electrophysiological studies and specific tests according to the suspected diagnosis. Having an etiological explanation aids in the development of a specific treatment plan; helps families understand prognosis and recurrence risk and on the community level assists in the development of preventive strategies.