Background: Professional Learning Networks (PLNs) of educators represent a promising approach to achieving continuous school improvement. At the same time, a range of studies in this area, including several systematic reviews and meta-analyses, report multiple conceptual and methodological challenges, or, at best, mixed results. Purpose and sources: The aim of this theoretical discussion paper, therefore, is to seek, first, to synthesise and connect to previous studies focused on professional learning communities and networks by combining and reflecting on their findings and recommendations. Second, we aim to contribute to the methodological development of the field in order to propose research that can link what happens in PLNs to changes in outcomes for students. For the latter, we will also make use of new insights from the field with regard to the use of big data in education. Main argument: We propose a conceptual model of what defines PLNs, enactment process variables, and influencing factors, presenting our theory-of-action for how PLNs can be effective. Second, we discuss challenges and recommendations in studying PLN impact regarding research approach, research design and measurement. This discussion includes consideration of the use of big data to help to make the analysis of patterns in, and relations between, different types of PLN research data more efficient and reliable. Conclusion: We need to define and study the processes and effects of PLNs more efficiently and effectively, to support PLNs in fulfilling the promise of increased teacher learning, improved outcomes for students, and, ultimately, sustainable school improvement at scale.