Psychologists are frequently asked to say whether, in the case of a particular patient, there is evidence of organic dementia. At first sight this would not appear to be an unreasonable request. A considerable amount is known about the performance of normal people on tests of cognitive function and it might be supposed that people who work with tests of human ability should be able to modify their procedures to the purpose of diagnosing intellectual deterioration when this is not obvious on routine neurological or psychiatric examination. Moreover, it might seem that the frequent presence of independent criteria of brain damage provided by neurological and neurosurgical investigations should facilitate the development of competent diagnostic clinical psychology in the neurological field.