Globally, there are ongoing problems with teacher retention, leading to a loss of experience and expertise. In policy and research, the emphasis is often on the professional development and retention of early career teachers, whereas teachers in later stages of their career are relatively under-represented. This article addresses this imbalance, reporting on a mixed methods scoping study that explores definitions of mid-career teachers in England and their retention and development, via a literature review, primary data collection and secondary analysis of data from the OECD's TALIS 2018 survey. We found that there is no agreed definition of mid-career teacher, relating to time in teaching, role and wider life circumstances and self-definition. Whatever definition is used, mid-career teachers are a heterogenous group, with varying needs, career plans and commitment to the profession. Whilst typically confident in their practice, their learning needs vary and are often experienced as unmet, especially for those looking for progression routes outside leadership and those with family commitments. This indicates that their potential for career development to benefit the profession may not be reached. The article concludes with suggestions for further study, policy and practice to improve understanding of this under-researched group.