The proposed research is concerned with the question of why is it that some problem solvers succeed on problems whereas others do not. This research is based on a theory of structural learning which gives explicit attention to the observer, the behaving subject, and the relationship between them. Rules which operate on other rules play a central role in the specification of the competence underlying given problem domains; postulated control mechanisms make it possible to explain and predict how knowledge is used to generate behavior and to create new knowledge; and, rules of competence provide a basis for measuring the particular knowledge had by individual subjects. Empirical support at each level already exists. In the cognitive domain, for example, a useful distinction is made between specific knowledge (rules and higher order rules) and universal control mechanisms which determine how available knowledge is put to use. In previous research, a "first approximation" control mechanism has been shown to accurately characterize the way in which (essentially all) seven and eight year old children use known rules to derive new ones (in problem solving). Traditionally assumed "consecutive application" control mechanisms, on the other hand, were shown to be inadequate. The proposed research is concerned primarily with: (1) the refinement, extension, and testing of alternative control mechanisms and (2) the possible emergence of such control mechanisms during the age span, four to seven years. In the former case, various types of problem solving processes (e.g., retrieval, rule selection) will be considered systematically: (a) to determine analytic/computational adequacy of the "first approximation" mechanism, (b) where inadequate, to devise parsimonious alternatives which account, both for previously tested processes and for new processes, and (c) conduct experiments to test mechanisms with respect to new processes. In the latter case, availability of control mechanisms will be tested with four-year-olds. Where lacking, we hope to determine the stage at which they emerge.