In recent years, teachers have turned to online social spaces for peer-to-peer interaction in increasing numbers. This online engagement has been highlighted by both practitioners and academics as having important implications for teachers' professional learning and development. However, there is a need to move beyond instrumental discourses that simply discuss engagement and technology in terms of costs and benefits, and analyse the complex social contexts in which engagement takes place. Therefore, presenting data from a digital ethnography of three online social spaces used by teachers, this paper uses professional identity as an analytical framework in order to understand teachers' online engagement in holistic terms in a way that acknowledges the messy social realities in which teachers work. It then presents a new theoretical framework for conceptualising teachers' professional identity that develops the concept of embedded ideal identity and takes into account context, social complexity, structure and agency.