This chapter focuses on teacher leadership, an important variable in the classroom and school improvement literature. The concept of teacher leadership has attracted increased attention in the past two decades. Teachers are assuming more responsibility for leadership roles and functions within schools. Despite the considerable amount of scholarly effort and time spent on investigating the teacher leadership concept, less is known about how it flourishes in the school context and how it relates to classroom and school improvement. Therefore, this chapter tries to shed some light on the teacher leadership concept and discusses its meaning, teacher leadership roles, factors influencing teacher leadership, the relationship between teacher leadership and classroom and school improvement, and future research areas on teacher leadership. Offering a framework for teacher leadership, this chapter is expected to contribute well to the guidance of further research on teacher leadership.