School classrooms can be conceptualised as bounded communities of practice made up of teachers and students working together to learn and build knowledge. The widespread use of information and communication technologies enables these communities to create knowledge, cross boundaries and build up intellectual capital. This paper, based on a qualitative study of thirty-two teachers in Victorian state schools, offers a model of four teachers’ roles that reflects the current situation, and suggests ways in which these roles might be developed to enhance knowledge building. It argues that safe, knowledgeable communities within boundaries, together with active boundary-crossing, can provide the conditions for knowledge building at classroom, school and system levels.