Transtornos do desenvolvimento intelectual are a group of etiologically diverse conditions originating during the developmental period characterised by significantly below average intellectual functioning and adaptive behaviour that are approximately two or more standard deviations below the mean (approximately less than the 2.3rd percentile), based on appropriately normed, individually administered standardized tests. Where appropriately normed and standardized tests are not available, diagnosis of Transtornos do desenvolvimento intelectual requires greater reliance on clinical judgment based on appropriate assessment of comparable behavioural indicators.