This article presents an analysis of two large-scale computer mediated conference environments, supporting teacher education programmes. The analysis draws on programme purposes and policies, log file data, user questionnaires, and conference observation. A review of the task of creating, developing and sustaining such environments for professional learning is presented, based in particular on the community of practice (COP) concept. This focus on a social view of learning is sustained in the final section of the paper, where a model is proposed for exploring the development of teacher professional knowledge. The model is exemplified by means of individual messages and discussion threads drawn from the conferences. The article concludes that professional learning in e-environments is only sustainable through the gradual development of joint enterprises, which draw on the common, as well as distributed knowledge, of a range of participants, including newcomers and expert practitioners.