According to Jaan Valsiner, development takes place within culturally structured environments jointly organized by the activities of children and the people around them. When overlap between promoted activity and the child's zone of proximal development exists, the structure of action that results from the interplay of the two is internalized by the child. This internalization constitutes development. Valsiner's perspective expands on Piaget's portrayal of structural genesis and change. Specifically, disequilibration may be considered as the result of asymmetries between the child's endogenous action structures and the exogenous, microgenetic (that is, occuring between the child and its environment) structures organized between the child and more capable individuals. Disequilibration, therefore, may be socially induced. Furthermore, it may be expressed in the child's sensorimotor and affective actions. These actions signal to others about need states. Caregivers may reorganize microgenetic transactions in ways that regulate the disequilibration. It is proposed, in contrast to Piaget, that the affective system, in addition to regulating the functioning of intellectual structures, may also modify the microgenetic structures that are internalized over time. This position is illustrated in a report of empirical observations of a 25-week-old girl who was videotaped for 12 weeks after she became interested in exploring and moving on and around a step which was located between two floor levels in her home. (RH)