Intellectual functioning is the product of an interaction among a variety of biological, social, and personal experiential factors. The distribution of ability at all levels is dependent on the manner in which the individual has learned to cope with this relationship of forces. This adaptive ability to cope is part of intelligent behavior. The concept of intellectual functioning can be conceived of as five separate, but interacting, areas: the sensory-motor development of the individual, cognition, the rate of learning, adaptive behavior, and social awareness. These continuously interacting elements must be considered in judging and assessing intellectual levels in all persons, including those with the types of atypical brain growth and development associated with mental retardation and developmental disability. Research tasks in this area are noted. (JDD)