Traditionally schools frame learning and the practice of teaching as the activities that go on within classrooms. In this arrangement, teachers usually are highly constrained in terms of their teaching activities and are increasingly seen as solely responsible for the learning of their students. In reality, the practice of education is invariably situated at the intersection of multiple Communities of Practice (CoPs). These include teachers' local and professional communities, the scientific communities related to the content areas of instruction (e.g. mathematicians, historians, etc.) and those associated with youth, parents, administrators, learning scientists and policy-makers, among others. Traditional forms of teacher professional development attempt to teach skills and provide opportunities to interact with other professionals, but this model is seen as weak and often very unsuccessful. In contrast, opportunities for teachers to expand the communities they belong to, enhance their own professional development and transform their own identities as mathematicians and teachers are better positioned to affect teaching and learning. The Math Forum is an interactive digital library that, since its inception in 1995, has focused on providing such opportunities for individuals to talk with others about math and work with others on math. As such, the Math Forum has become one of the premier sites on the Internet for math education. This chapter focuses on the ways that Math Forum has supported multiple overlapping CoPs among teachers and the ways that teachers have used the Math Forum to expand their sense of membership and interaction with diverse CoPs. Recommendations for CoP practitioners are outlined as well as further questions for research.