Previous research has shown that teachers' informal learning in social networks is of great importance for their professional development. Participation in social networks is often characterised by knowledge exchange between employees with the aim of dealing with work-related challenges or improving the work practice. In this study, attention was focused on teachers at a Swedish police education unit and their participation in work-related social networks. The findings were based on two data collections, logbooks of social networking interactions and follow-up interviews with 23 teachers. The purpose was to explore who they talked to, for what purpose and what means of communication they used, and the analysis included mapping of their interactions in social networks and a qualitative analysis of their narratives. Overall, the study shows that the teachers' participation in social networks was extensive and valuable for their professional development as teachers. Furthermore, the analysis shows that the teachers' interactions in social networks are usually intentional and based on knowledge needs related to pedagogical aspects of teaching, that the composition of course content and teaching teams can affect teachers' readiness to participate in several different networks and that the type of knowledge need affects which means of communication are used.