A four-year longitudinal study of concept attainment and uses of concepts, as specified by the Conceptual Learning and Development Model, is described. Data collected during the first two years were based upon the assessment of the concept, equilateral triangle. The assessment battery was administered to 351 children (grades K, 3, 6, and 9) in 1973 and 1974, to two control groups, and to a different sample of 275 children. Age changes in children's conceptual development were predicted according to the model, and were strongly supported by both sets of data. The concept was attained according to the four sequential levels: concrete, identity, classificatory, and formal. Children who had attained the concept at the formal level were increasingly able to use the concept in cognizing supraordinate-subordinate relationships, understanding principles, and solving problems. Progression in mastery of higher levels of concept attainment was associated with ability to apply the concept for the three uses. Participation in the assessment of level of attainment had no biasing effect on either level of attainment or uses of the concept. The resulting data are presented. (Author/GDC)