Teacher leadership is increasingly being seen as a key vehicle for school improvement and renewal. However, research on this phenomenon is limited, especially outside of the US. This article presents findings from an empirical study of teacher leadership in the UK, aimed at exploring both the ways in which teacher leadership can influence school and teacher development, and what in-school factors can help or hinder the development of teacher leadership in schools. The study was undertaken using a qualitative case study approach, purposively selected as being sites where teacher leadership was operational. Data indicated that teacher leadership was characterised by a variety of formal and informal groupings, often facilitated by involvement in external programmes. Teacher leadership was seen to empower teachers, and contributed to school improvement through this empowerment and the spreading of good practice and initiatives generated by teachers. A range of conditions needed to be in place in schools for teacher leadership to be successful, including a culture of trust and support, structures that supported teacher leadership but were clear and transparent, strong leadership, with the head usually being the originator of teacher leadership, and engagement in innovative forms of professional development.