This research provides a less mechanistic lens in which to consider teachers’ PD. It is argued that, essentially uncontrollable forces of community, identity and practice can be designed for, and ultimately sustain teacher engagement. This longitudinal study of Australian and United Kingdom teachers uses Wenger’s theory of Community of Practice as both a design and analytical framework for Professional Development (PD). A model of community cohesion is proposed which recognises the need for teachers to explore their identity which is intrinsically connected to their practice. They need community brokers to help them to shift their trajectories into increasingly centripetal practices, and fundamentally they have to engage with other members in mutual, accountable and negotiable ways. During the course of this research, two groups of teachers participated in a face-to-face training day followed by a minimum four weeks online PD. However, both case studies reported engagement in the PD for more than twice this period. In addition, results suggest that sustainability was supported through community cohesion. In particular, participants’ valued social interaction as a way of negotiating mutuality of engagement, reported accountability to joint enterprise despite issues of critical mass, and shared repertoire as a way of negotiating community membership.