Children's physical growth is usually not striking to people who live with them day in and day out. Development, whether physical or mental, unfolds gradually and naturally over a long period of time. The fact that development occurs gradually, however, does not necessarily mean that the rate of development is always the same. There are times when a child appears to be learning at faster or slower rates. Different rates of both physical and mental development have been documented throughout childhood and adolescence (Emde and others, 1976;Fischer, 1980;McCall, 1979; White, 1970). Some researchers suggest that new learning occurs mainly during the faster periods of physical growth (Epstein, 1974), with skill acquisition progressing rapidly during certain high growth periods and then leveling off during slow growth periods. The approach presented in this chapter does not draw quite so strong a dichotomy between fast and slow periods of development. Instead skills are viewed as accumulating in an ongoing and continuous process, with spurts in developmental pace marking the emergence of new learning potentials.Development is therefore both continuous and discontinuous. Individuals experience gradual transitions and stage-like changes in the process of skill acquisition. Neither aspect can be ignored without losing a piece of the