Cognitive development during each of the major stages identified by Piaget is characterized by abilities to solve progressively more complex tasks (e.g., changes in the object concept during the sensori-motor stage, and in conservation of amount, weight, and volume during the concrete operational stage). Several theorists have suggested that these changes could be explained by increases in working memory or information processing capacity. Case (1978) suggests that development within each of the major Piagetian stages reflects the growth of working memory for the class of cognitive operations which become available to the child in the stage. The hypothesis that working memory for "formal" cognitive operations increases from one at ages 10 to 12 to four at ages 17 or 18 is examined. Evidence from prior studies of formal operational thought is reviewed. Developmental data using several measures of formal operational memory including tasks involving anticipation of elements in a classification matrix, solution of algebraic problems, and ratio relationships is presented. In general, results are consistent with the hypothesis that there is development of a formal operational working memory. Implications for instruction and other aspects of adolescent development are discussed. (Author)