This thesis explores the relationship between preschool teachers’ professional competence and their pedagogical practices. Specifically, preschool teachers’ considerations on two pedagogical practices are studied: (i) organizing children into subgroups and (ii) working with curriculum content areas. The research is part of a Swedish research project entitled “The Impact of Group Size on Children’s Affordances in Preschool”, financed by the Swedish Research Council. Ecological systems theory is the theoretical framework of the research. The thesis includes four empirical studies. The analyses are based on data drawn from preschool teachers’ responses to questions included in the project’s web-based questionnaire. In study I, preschool teachers’ professional profiles across different preschools in Sweden are explored. The results show differences between profiles with respect to preschool teachers’ graduation year, continuing professional development, and experience. Studies II and III aim to generate knowledge about preschool teachers’ practice of organizing children into subgroups. The results from these two studies put forward the organization of children into subgroups as a collectively negotiated practice embracing a child-centered perspective. Study IV focuses on preschool teachers’ considerations on the curriculum content areas they least involve in their daily work. Reading and writing are revealed as the least-emphasized content areas. The combined results of the four studies point to the importance of addressing the diverse needs for professional development among preschool teachers, along with the challenges, dilemmas, and questions embedded in their immediate environment. A transactional approach to preschool teachers’ professional competence, which considers preschool teachers’ diverse competences along with theoretical and practical knowledge, is suggested. Parts of work Study I: Nasiopoulou, P., Williams, P., Sheridan, S., & Yang Hansen, K. (2019). Exploring preschool teachers’ professional profiles in Swedish preschool: A latent class analysis. Early Child Development and Care, 189(8), 1306–1324. visa artikel Study II: Nasiopoulou, P. (under review). Decision-making factors for group organizing in Swedish preschools. Study III: Nasiopoulou, P. (2019). Investigating Swedish preschool teachers’ intentions involved in grouping practices. Early Childhood Education Journal. visa artikel Study IV: Nasiopoulou, P., Williams, P., & Lantz-Andersson, A. (submitted). Preschool teachers’ work with curriculum content areas in relation to their professional competence and group size in preschool: A mixed-methods analysis.