This paper presents the major findings of a longitudinal study of teachers' professional identities in the early years of teaching. It analyzes key influences upon the ways in which new teachers' identities are shaped and reshaped over time. Through their own perceptions, analyses of the school cultures in which they work and their pupils' views it reveals how the interplay between contextual, cultural and biographical factors affects their teaching practices. Teachers' personal and professional histories and pre-service training, alongside issues of school culture and leadership, emerge as stronger mediating influences (than previous literature suggests) in determining the kinds and relative stability and instability of professional identities which teachers develop in the early years of teaching and thus the kinds of teachers they become and their effectiveness.