In the current review, we examined teacher leadership research completed since York-Barr and Duke published the seminal review on teacher leadership in 2004. The review was undertaken to examine how teacher leadership is defined, how teacher leaders are prepared, their impact, and those factors that facilitate or inhibit teacher leaders’ work. Beyond this, the review considered theories informing teacher leadership, teacher leadership within disciplinary contexts, and the roles of teacher leaders in social justice and equity issues. The most salient findings were (a) teacher leadership, although rarely defined, focused on roles beyond the classroom, supporting the professional learning of peers, influencing policy/decision making, and ultimately targeting student learning; (b) the research is not always theoretically grounded; (c) principals, school structures, and norms are important in empowering or marginalizing teacher leaders; and (d) very little teacher leadership research examines issues of social justice and equity.