Consensus is growing that teacher leadership benefits teaching quality and student performance. Despite the recognition that teacher leadership contributes to teachers' professional development, little is known about how it is developed and how teachers experience the transition to the teacher-leader role. This study explores the internal mechanisms underlying the transition to and formation of teachers' professional identity as teacher leaders. It is based on 60 interviews: 41 teachers who were selected to participate in a leadership training programme, 10 principals and 19 teacher-leaders' colleagues. The findings led to a model with four central components: (1) Overall professional identity; (2) The experience of 'being chosen;' (3) An internal meaning-making process; and (4) External forces.