Drawing on findings from a national study of learning-focused leadership in challenging urban settings, this article examines the work of teacher leadership in urban high schools. In this context, a recently emerging cadre of nonsupervisory teacher leaders, working in collaboration with supervisory leaders, exercises a form of "distributed instructional leadership," creating new channels for instructional renewal. Extensive qualitative data from four high schools in different urban districts across the United States illuminate the nature of these teacher leaders' work in support of teaching and learning, the ways they and their supervisory counterparts worked as instructional leadership teams, and the role that they played in supporting teachers in classrooms within schools that face significant challenges. Study findings highlight the unique expertise necessary for these teacher leaders--including content expertise, pedagogical coaching skill, ability to build relational trust, and capacity to link the classroom with district- and school-determined learning improvement efforts. In conclusion, the article sketches the implications for reconfiguring supervisory leaders' work so that the efforts of all remain productively and persistently focused on learning improvement.