Professional learning communities are promising for teacher learning and improving the quality of education. In the past decade, there has been a shift in focus from within-school to cross-school PLCs: Professional Learning Networks. Knowledge of the underlying working processes of teacher learning in PLNs is scarce. This is even more complicated for PLNs, because of organizational, geographic and professional boundaries. The purpose of this study is to explore how PLNs can function to meet their objectives and what challenges they face in their aim of achieving sustainable teacher learning and school improvement. We applied a qualitative case study design. Data were collected by means of interviews with PLN participants and external coaches, logs by external coaches, and yearly observations by the researchers. Results show how the PLN context influences the outcomes in terms of teacher learning, PLN products and application in their schools. Findings show how leadership, the perception of a shared goal, structured activities, collaboration, and facilitation and support especially appear to influence outcomes. PLNs face additional challenges in relation to within-school PLCs, in terms of knowledge transfer from the PLN back to participants' own schools and the network as a whole. This should be acknowledged and acted upon before PLNs are even established.