This review aims to clarify the concept and boundary of research on online learning communities, identify the major trends of research, and suggest pertinent issues for future research. Four online learning communities, namely, Knowledge Building communities, Quest Atlantis, Virtual Math Team, and Web-based Inquiry Science Environment were reviewed. We compare and contrast the design principles and characteristics of these learning environments along cognitive, social, and technological dimensions. Potential implementation and research issues were discussed, including the contradictions with traditional school cultures and practices, the issues of authenticity versus simulation approach in schools, the cognitive and socio-emotional outcomes of online learning communities, and the possibilities and impact of advances in technologies.