This paper reports findings from a study of support provided by non-school-based mentors of secondary science teachers in England. It focuses on the identity development of beginning teachers of physics, some of the recipients of the mentoring. Drawing on the analysis of interview and case study data, and utilising third space theory, the authors show how external mentors (experienced, subject specialist teachers who were not based in the same schools as the teachers they were supporting) facilitated opportunities for mentees to negotiate and shape their professional identities, and made valuable contributions to three distinct and important aspects of beginning teachers' identity development. The paper argues that non-judgemental support from external mentors enhances beginner teachers' professional learning and identity development through the creation of a discursive 'third' space in which mentees are able to openly discuss professional learning and development needs, discuss alternatives to performative norms and take risks in classrooms. Opportunities for beginner teachers to engage in such activities are often restricted in and by the current climate of schooling and teacher education within England.Keywords: beginning teachers; external mentoring; teacher identity; teaching physics; third space Introduction This paper is based upon an analysis and theorisation of a subset of data generated for an original study of external mentor support for teachers in England (Hobson et al. 2012). We use the term 'external mentor' to refer to an experienced teacher who has the same subject specialism but is not employed in the same school as the teacher they are supporting. The interaction between external mentor and mentee may take place within and/or outside of the mentee's school, and may be face-toface and/or remote. The data analysed relate to mentoring support on two programmes. First, a pilot programme of regional mentoring for participants undertaking the Physics Enhancement Programme, a subject knowledge enhancement (SKE) programme for non-specialist beginning teachers of secondary physics (Shepherd 2008 'how teachers define themselves to themselves and to others ' (Lasky 2005, 901). In particular, we show how working with a subject specialist educator operating in a purely supportive role can enable beginning teachers of physics to overcome challenges associated with three important features of identity development. In so doing, we suggest that 'third space' (Bhabha 1990) provides a valuable theoretical lens to 10 understand how external mentors facilitated the emergence of new dialogic spaces, within which the beginner teachers were enabled to better understand and negotiate their professional identities. Understanding how pre-service and early career teachers shape and can be helped to develop their professional identities is an important consideration to all who work with them, not least because of the interplay between tea-15 cher identity and teacher resilience, teacher well-being and teacher effective...