Communities of practice, in which learning and teaching are interwoven in social networks, may someday lead to a movement to put thoughtful professional expertise back into schooling. ********** DURING my first month of graduate school, I read that Japanese teachers' school days included time to discuss their practice and plan lessons with their colleagues. Many years have gone by since I came across that information, but sometimes I still think about it. The reason that image of teacher collaboration made such an impression on me, I think, is because I had recently left elementary school teaching after only a few months as a long-term substitute teacher. An important factor in my decision had been that I felt so isolated from other teachers who might have goals similar to mine. The experience of isolation was especially profound because I had just graduated from a small, alternative teacher preparation program called that was based on social theories of learning. The program practiced what it preached. We preservice teachers were engaged in learning in the social context of a community; we studied and discussed practice together, co-taught, supported one another, formed friendships, and developed ongoing collaborations and conversations with colleagues with similar interests. Less than one year after graduating from the Learning Community program, I was in my first teaching job--alone and starved for the kinds of connections I had had and the ideas and support that they had provided. It was only in hindsight, once I began to study teacher networks as a researcher, that I recognized why I was so struck by that article about teaching in Japan. Researching school change in an urban middle school in Philadelphia, I met Jennifer and Ellen, two teachers who were exceptionally well connected to networks of educators drawn together around common interests. These interests included multicultural education, community service learning, teacher research, teaching writing for English-language learners, and more. I was struck by the way Jennifer and Ellen were revitalized by meeting outside the school day with other educators who shared their passions. They were excited by the new ideas that they brought to the school from their networks, and they could trace their own changing practice in terms of those ideas. As was the case for the Japanese teachers and for me in Learning Community, for Jennifer and Ellen, learning and teaching were interwoven in social networks. Since the completion of my research project, I have read the growing literature on teacher professional development networks and have learned that Jennifer and Ellen are not alone. In recent years, teacher networks--defined here as groups of teachers organized for purposes related to teacher learning, inquiry, support, or school improvement--have been embraced by researchers and practitioners alike for their approach to teacher professional development. (1) In contrast to such traditional professional development approaches as workshops or inservice days, networks often reflect a social or constructivist orientation to teacher learning. Many are based on the premises that contexts for teacher learning should endure over time, build on teachers' knowledge and experiences, provide opportunities for critical dialogue and inquiry, and foster the public sharing of practice and understandings. Turning to learning communities outside of their schools, teachers can find everything that schools, too often, are not. NETWORKS AS COMMUNITIES OF PRACTICE Researchers and advocates of teacher networks have suggested that one of the reasons that there is so much excitement about this approach to teacher professional development is that networks are examples of communities of practice, (2) a concept developed by Jean Lave and Etienne Wenger. (3) In my view, the power of this idea is in the conceptualization of learning-as-social-participation, which begins with the assumption that engagement in social practice is the fundamental process by which we learn and so become who we are. …