This article presents a systematic review of a substantial body of literature that considers the ways in which the concept of teacher identity has been used to understand and explore teachers' professional lives. The aim of the review was to go beyond the limitations of specific areas of teachers' practice to explore the broad and rich field of teachers' professional lives. Drawing on 412 articles from 2000-2021, the review demonstrates the growth in teacher identity research, particularly since 2010. Results from the review are categorised into seven thematic groups that span key areas related to teachers' professional lives: (1) Models and frameworks of professional lives; (2) Narratives of professional lives; (3) Becoming a teacher; (4) Contexts; (5) Communities; (6) Change, transition and conflict; and (7) Subject specialisms. The review reveals gaps in teacher identity research, such as: few research studies from Global South countries; a limited number of studies focusing on non-core curriculum subjects, including arts, history, geography and physical education; fewer studies focusing on primary school and early years teacher identities; and a divide between identity scholarship and research, and policy and practice. The article concludes with a call for teacher identity research to be expanded and broadened with the aim to facilitate the progress of identity work in scholarship, policy and practice.