The purpose of this article is to suggest a conceptual framework for understanding professional communities of teachers, specifically those communities that extend beyond workplace, and in some cases national, boundaries. The nature of the paper is a theory-building paper informed by a review of literature in this field. The literature that informed this paper is of two types: firstly, case study reports written by practitioners engaged in professional communities; secondly, academic papers that explore the nature of such communities. The use of the first of these genres means that the framework presented emerges from the experiences of teachers and other practitioners, rather than being imposed upon them. This article presents a conceptualisation of five aspects of professional communities. These are as follows: the ways in which teachers’ self-efficacy may be shaped by their perceptions of membership of a professional community; the boundaries of such communities; the challenges posed by recognising community membership, and also by individuality; and in the final section, the potential for knowledge-sharing within such communities. Its contribution to the wider academic debate is its potential to inform empirical research on communities that is currently taking place, by means of a wide range of projects, in universities across Europe and beyond.