This article explores four roles of teachers in classrooms using computers, from the perspective of communities of practice (Wenger, 1998). It reports on an indepth study undertaken in 12 schools, and shows that teachers appropriated technology in a range of ways to help them create classroom communities that build knowledge. Some also acted as brokers to cross classroom and school boundaries, engaging in professional learning through curriculum projects with other teachers and their students as new communities of practice formed. However, while such projects were initiated and driven by individuals and groups of teachers, their success required support through school leadership and organization and statewide technology infrastructures and funding.