In order to obtain a fair indication of the value of intelligence tests in their relationship to mentally defective children, it will be necessary to consider the tests from several view-points: it is proposed to deal with the system: (a) as a means of diagnosing mental defect; (b) as a means of grading mental defect; (c) as a guide to prognosis; and (d) in its relation to treatment. We shall then proceed to take up the test reactions of some definite clinical groups of defectives , and finally consider the significance of a few individual tests. The Tests employed, with some observations on testing technique in cases of (pronounced) mental defect. The tests employed were these of the Stanford Revision of the Binet-Simon Tests , as described in Terman's "The Measurement of Intelligence" , revised for application to British children . Binet-Simon tests were used in preference to any other scale, because: ( I ) they are still the tests very commonly used* (II) they have been carefully standardised, (III) they were specially designed to deal with the subnormal child, whereas some of the other tests are meant to detect rather children of superior mentality. In the present investigation it was not found necessary to use tests above year 16. A list of tests used is appended.