The purpose of the study was to investigate the dynamics of physical activity (PA) in day care and preschool. The participants were 823 Finnish 1-7-year-old children from 50 day care centres and preschools. The research methods were systematic observation, evaluation of children's skills and interviews with children. Altogether 18,366 observations were collected. Vygotsky's concept of the 'zone of proximal development' is redefined in relation to PA. The results of the study show that environmental factors, and specifically peers, do indeed play a significant role in enhancing PA among children. There are cogent reasons for paying more attention to the dynamics of children's PA. Furthermore, there is reason to ponder extensively how to provide opportunities for PA and, more precisely, the development of motor competence in the day care and preschool context. (author abstract)