Two assumptions underlying preschool programs for high risk students are examined: the belief that measurement instruments are available for diagnosing children who will eventually require some form of special education and that intervention will be beneficial to the child. Diagnosis favors teacher interaction with the student in an instructional setting over individual instrument evaluation. Intervention involves a variety of aspects which must be considered in determining effectiveness. These include the delivery agent (home based or school program) and growth dimension (academic achievement or cognitive ability). Cited studies support the validity of early assessment and the effectiveness of intervention programs at the preschool level. (CL)