Considering how much time most children spend in Early Childhood Education and Care (ECEC) institutions, the psychosocial dimensions of ECEC institutions and children's well-being represent a growing research area within the ECEC field. This paper presents results from a study where children's experiences of subjective well-being and opportunities for participation in ECEC institutions were explored. Research on children's own perspectives about their well-being has mainly been conducted among children older than those of preschool age, and therefore this study aimed at highlighting the voices of 4-6-year-old children regarding how they experience their lives in ECEC institutions. Quantitative data was collected through conversations with 171 Norwegian 4-6-year-old children based on an electronic questionnaire. The results indicate that even though many children experience a high degree of well-being in Norwegian ECEC institutions, approximately one-third of them experience a medium level of well-being and close to one out of ten experience a low degree of well-being. The results show that the physical environment, the toys/equipment, the common activities and the opportunity to have an influence on where to move, what to do and with whom are of crucial importance for children's well-being in ECEC institutions. This also includes children's opportunity to oppose the staff and negotiate and choose activities that differ from those that the staff select. (author abstract)