Professional learning communities (PLCs)--teams of educators who get together regularly to exchange ideas--have sprung up to meet school districts' growing interest in promoting professional development that engages teachers and administrators. PLCs meet to develop lesson plans, monitor student progress, assess instructional effectiveness, and identify professional learning needs. The ultimate goal is to raise student achievement by adapting teaching and classroom practices based on learning and interchanges during PLC meetings. The Internet and mobile communication technologies have greatly expanded opportunities for teams of educators to reflect and collaborate with each other and experts outside their schools--and even outside their districts--for learning, joint lesson planning, and problem solving. These electronic platforms provide ready access to knowledge and resources without the usual limitations of time, space, and pace. Hybrid PLCs combine online interactions with the face-to-face interactions of traditional PLCs. This review of the literature on online PLCs responds to a request from district and school administrators in the Regional Educational Laboratory Mid-Atlantic Region for information on using online PLCs to engage their teachers in professional development. The review looked at advantages, challenges, and emerging best practices. [This report was prepared for the Institute of Education Sciences (IES) by Regional Educational Laboratory Mid-Atlantic administered by ICF International. For the full report, see ED544210.]