Simple clinical testing procedures, devised by Sheridan, to test visual functions in young and/or handicapped children are discussed. Using these methods, combined with experience obtained in studying visuo-motor behaviour in cats, vision was tested in 30 severely mentally handicapped children. For 17 of these, it was unclear whether they possessed any usable visual functions. For 29 out of the 30 tested children it was possible to reach a reasonably confident conclusion regarding presence or absence of functional vision. Thirteen patients appeared to be functionally blind. In 17 children the size of the visual field could be determined; 7 showed normal, 10 restricted visual fields. In 6 children acuity could be measured. The importance of early detection of visual defects is discussed.