Research is cited to corroborate the lack of conceptual models for building a functional psychological evaluation system for developmentally disabled persons. A model is proposed for individualized evaluation/assessment using a functional approach and based on the cognitive-behavioral perspective. The model can be used to determine directions in service plan delivery. Assessment is differentiated from evaluation. Guidelines are offered for six stages in the psychological assessment process (sample subtopics in parentheses): screening (reason for referral); evaluation (evaluation of competencies and incompetencies for Individualized Service Plan); assessment (diagnostic considerations, identification of additional services); program plan and design (goal oriented statements); curriculum development (educational, vocational, self-help, recreational); and program implementation (inservice and staff development). A concluding section provides background information and clinical observations about psychological assessment of intellectual functions, affective disorders, physical and behavioral disorders, and the role of assessment as an integral part of the treatment process. (CL)