This report examines the efforts of school administrators and teacher leaders to improve instruction at 15 well-functioning, high-poverty schools in four urban districts. It describes how effective school leaders react to external demands (including district expectations and state and federal policies) and external supports (such as district professional development) in pursuing instructional improvements. It also details the efforts of administrators to improve instruction, such as their approach to hiring skilled teachers, removing incompetent ones, supporting teacher leadership and promoting teacher collaboration. The report then examines the variety of roles for teacher leaders, who may serve as classroom coaches or professional development leaders. It also looks at challenges they face as non-supervisory leaders and as go-betweens for school administrators or the district. Based on the evidence, researchers describe ways administrators and teacher leaders can best pursue their work. Principals, for instance, need to create a school culture where teachers feel safe in having their classroom practices critiqued. Teacher leaders, meanwhile, must know how to build trusting relationships with colleagues in order to guide them towards improvement.