AbstractIt remains a major challenge to find appropriate ways to use the Internet for teacher professional development. We look carefully at one widely-used analytical construct, communities of practice (CoP). We present a review of the research along with original research that highlights the need to understand the many overlapping live and online communities of practice a teacher may belong to. We revisit the original work of Lave and Wenger (1991) and highlight crucial structural features of CoPs. We give several examples of overlapping on-line and live CoPs. We conclude by describing a design experiment which attempts to align the structure of the online community with live communities.