There are two major problems that Piaget's work poses for modern psychology: the problem of discontinuous development via qualitative stages, and that oflearning and stage transitions via equilibration (see Pascual-Leone, 1980). Only the first problem is fully recognized. Most developmentalists accept that some form of qualitative stages/levels of processing exists. These stages emerge in spurts in correlation with age, but are not bound to age in any simple mannerat least when stages are formulated as general structures (i.e . , defined across types of situations for a type of subject) . The hierarchy of stages is often expressed by means of a psychological series of models, Piagetian or neo-Piagetian, each describing the competence of one stage and, indirectly, showing the processing complexity that the child of this stage can handle.