The experience of headteacher socialization has been described as challenging and often traumatic for new headteachers. The research reported in this article provides a theoretical explanation of that experience by analysing the socialization of new primary school headteachers in England from a role boundary perspective. The role boundary is the delineation between a set of practices--a role-as-practice--which are commensurate with a role-as-position and those practices that are not. Six newly appointed primary school headteachers were interviewed periodically during their first three years in post to explore their experiences. A seventh newly appointed headteacher kept a written log of his experiences during that period. From the data, 22 narratives of critical socializing incidents were developed and interpreted from a role boundary perspective. During headteacher socialization, the new headteacher and those in the school and in its wider context seek to establish and position their respective role boundaries. Role boundary interactions have the purpose of establishing the legitimacy of practices and may be immediate; intense; lead to conflict and have a high affective content. The research confirms the role boundary concept as a theoretical and methodological tool of heuristic and analytical value in understanding and explaining headteacher socialization.