Background Study: Previous systematic reviews concluded that online professional-learning communities (OPLCs) have positive effects on teachers' development, but explored neither the technological affordances that make this possible nor the potential negative impacts of technology use in OPLCs. Objectives: The aim of this study is to systematically analyse previous research on teachers' OPLCs, and thus help answer the question of what technological affordances (dis)empower members of teachers' OPLCs, and how they do so. Methods: This study applied the systematic-review method. A total of 28 studies were identified and analysed using rigorously defined inclusion and exclusion criteria. The results are reported as a synthesis. Results and Conclusions: First, in theme, the selected articles primarily focused on features of OPLCs and of teachers' knowledge. Second, mixed-methods approaches were commonly used in the sampled research, and the number of studies employing advanced statistical methods and machine learning increased over time. And third, the reviewed studies' findings suggest that technology supports 'bottom-up' teachers' communities (as distinct from 'top-down' professional development organized by educational authorities) in seven ways: that is, by promoting collaboration opportunities, expanding the participants' networks, facilitating knowledge sharing, delivering support more rapidly, providing emotional support, allowing ubiquitous learning, and enabling multiple modes of participation.