This monograph reports the proceedings of a symposium that considered issues in the use of learning hierarchies in both psychological and educational research. The opening paper presents a brief overview of research on learning hierarchies. Issues considered include the use of behavior analysis as a basis for generating hierarchies, the extent to which hierarchy theory can take into account individual differences in learning patterns, and the implications of cumulative learning of increasingly more complex tasks for theories of cognitive development. Different methods of validating hypothesized hierarchies are discussed. In the succeeding papers, four instances of research on hierarchies are presented, each representing a different approach to the problem of hypothesizing and testing hierarchical relations among cognitive tasks. Several discussants, representing the developmental, learning, and psychometric points of view, then consider implications of the research reported. (Author)