Learning communities are best defined as "ongoing groups who meet regularly for the purposes of increasing their own learning and that of their students." Although learning communities vary in form and context, they share some fundamental core beliefs and values. Based on the idea that educators can learn from each other, learning communities create and maintain an environment that fosters collaboration, honest talk, and a commitment to the growth and development of individual members and to the group as a whole. They work from the assumption that teachers are not mere technicians who implement the ideas of others, but are intellectuals who are doing knowledge work. This means that learning communities privilege theory as well as practice; they encourage and support members to examine their practice, to try out new ideas, and to reflect together on what works and why; and they provide opportunities for the collective construction and sharing of new knowledge. Equally important to the concept of a learning community is the connection it forges between professional and student learning. As educators identify and solve problems of practice together, they build the capacity and collective will to move forward the equity agenda of their schools and districts and enhance the learning and achievement of all students. In this article, the authors present five research studies that bolster the case that learning communities in schools are a critical element in professional development and student achievement. Each of these studies has made a major contribution to a growing and powerful research base about learning communities, how they can transform classroom practice, and, ultimately, enhance student learning. The authors describe each study and how it adds to one's understanding of what learning communities are, what they do, and how they develop.