Clinicians often assume, in a somewhat casual manner, that injury to the brain at birth has been the cause of this or that case of defect. Cerebral diplegia, hemiplegia, hydrocephaly and numerous types of cerebral maldevelopment have been attributed to this cause. Some American psychologists have tended to ascribe to birth injury every case of defect with plegia (Doll, Phelps and Melcher, 1932). Equally categorical denials of the influence of birth trauma in the aetiology of mental defects have been expressed (Berry, 1933). The decision between these opposing points of view involves the examination of some difficult questions. There are two rather obvious principles to be applied. First, any case in which birth injury is diagnosed must present symptoms or signs, which could reasonably be attributed to this cause. Secondly, a history of events at the time of birth must be ascertained and the evidence carefully weighed with a view to finding out whether conditions likely to produce injury were present. A third point to be kept in mind is that, even when proof of cerebral injury at birth is reasonably complete, the effect of such injury on the intelligence level has still to be assessed. In any given case of defect with a possibility of birth injury, we must ask ourselves what the intellectual level would have been in the absence of such injury.