The effective implementation of positive behaviour intervention and support (PBIS) requires both organisational and professional change. This means that the whole community including all educators and children commit to shared principles, practices, and structures to create a safe and supportive social climate. In this article, we investigate the initial implementation of the Finnish PBIS ProVaka in eighteen early childhood education and care (ECEC) centres. Our inductive qualitative analysis of longitudinal progress diary data focuses on roles, responsibilities and communication among the ECEC communities. The results show that successful ProVaka development is a matter of successfully expanding the actor network and shared responsibility. The challenges of implementation are related to 1) an equal knowledge base, 2) sufficient concretisation of ProVaka, 3) a balance between theoretically valid and culturally adapted implementation, 4) regulation of motivation and enthusiasm, 5) the constant need for change and stances towards it and 6) the clashes of everyday life realities and implementation. This actor- and communication-focused inductive qualitative study offers new insight into the initial implementation phase of PBIS in Nordic ECEC and can inform the implementation and development of other evidence-based methods in special education.