Theory and research regarding four levels of concept attainment and three uses of concepts as specified by the conceptual learning and development (CLD) model are described. Assessment batteries were developed to assess each child's level of concept attainment and also the related use of the concepts equilateral triangle, cutting tool, and noun. Each of the three batteries was designed as a paper-and-pencil task and administered to from 60 to 100 children enrolled in each of four grades: kindergarten, third, sixth, and ninth. The three populations receiving the three batteries were not independent; more than 80 percent of the children received all three batteries. Predictions based on the model about children's conceptual development were strongly supported across all three concepts: (1) The three concepts were attained in an invariant sequence according to four successive levels: concrete, identity, classificatory, and formal. (2) As the concepts were attained to higher levels they were used increasingly (a) in cognizing supraordinate-subordinate relationships in a hierarchy where the attained concept was an element of the hierarchy; (b) in understanding principles that stated a relationship between the attained concept and one or more other concepts; and (c) in solving problems that required the use of the particular concept. (3) Having the labels of the concept and of its defining attributes facilitated attainment of the concept, and mastery of the three uses of the concept. (Author)