The topic of differential diagnosis and its application to the use of the Revised Illinois Test of Psycholinguistic Abilities (ITPA) and Wechsler Intelligence Scale for Children-Revised (WISC-R) is discussed. The theoretical bases for differential diagnosis and the properties of assessment instruments with diagnostic properties are presented. Statistical procedures for determining whether tests have differential diagnostic power are discussed and illustrated by using the American and Australian standardization data from the ITPA and the American standardization data from the WISC-R. (Author/RC)