In this survey paper we approach the challenge of scaling CPD from four perspectives. First, we elaborate on crucial aspects of teacher learning and what taking the learning of these crucial aspects entails. Second, we focus on different CPD frameworks to showcase developments in CPD research and practice over the last 40 years and the influences of different views of CDP. Third, we elaborate what developing CPD in an evidence-based way means, before we finally discuss crucial issues of spreading CPD on a large scale. In this last perspective, we draw on Coburn's four dimensions characterizing the process of scaling CPD interventions, depth, sustainability, spread, and shift in reform ownership to discuss how the challenge of scaling high-quality CPD might be successfully addressed. Our literature review is based on findings from education research in general and mathematics education research in particular in order to pay attention to both overarching aspects of providing effective CPD and the domain-specific factors of mathematics teaching and learning. Against this background, we identify needs for further research and provide an overview how the papers in this Special Issue of ZDM might address these needs.