Teachers' professional lives are situated at the intersection of local, national and global educational policy contexts. What they purposefully do (agency) and how they see themselves and their roles as teachers (identity) dynamically interact with such contexts. This study argues that in order to understand the meaningful professional development work of teachers, it is important to have an understanding of this interplay. Current dominant policy discourses concerning the 'improving teacher' and 'teaching as a craft' are examples of an over-reliant emphasis on more insular narratives of agentic teachers and teaching. As the research in this thesis shows, such narratives fail to take into account the complexities of factors and discourses that impact on the beings and doings of teachers, and are therefore inadequate. Based on an iterative dialogue between particular theoretical ideas and emerging case study data, the study proposes a multi-level integrating framework for understanding the experiences of teachers as they develop and locate a sense of their professional identity. Four teachers, from different types of English secondary schools, participated in the study. Data was generated from timelines, concept maps, lesson observations and interviews with the teacher participants. The case studies were presented as written portraits. Drawing on Archer's work (e.g. 2012) on reflexivity, the ways in which teachers' thinking mediated the links between their agency and structure were considered. The different modes of reflexivity that teachers employ and the ways in which teachers determine and facilitate personal projects of concern to them were found to be important to their professional identity and agency. The findings also suggested that the similarities and differences between the teachers were to do with how intersecting structural and cultural factors at global and local levels are mediated by individual forms of reflexivity. These forms of reflexivity are a reflection of evolving personal and social identities and an emerging social stance on society. The mediation produces particular professional concerns or projects that both suggest similarities that relate to powerful global discourses of education-such as performativity-but also particular types of agency and identity that are specific to those individual teachers' classrooms and general professional stance. The essence of the daily work of teachers appeared to reflect an intersection of personal biography and the situational structures and cultures of schools in which teachers operated, which brought about differences in professional thinking and doing. The thesis contributes to knowledge by adding to theory concerning identity and agency, as well as contributing to methodology by using portraits in understanding the nature of teacher agency and reflexivity. The factors that are identified and an insight into teachers' reflexivity contribute to the development of a toolkit for understanding teachers' identity and agency that may be useful for both teacher educators and policy makers.