This report has two main purposes: to examine the perceived impact of teacher leadership, and to explore the conditions in schools that support or constrain teacher leadership. It discusses peer-nominated, nonadministrative leaders; leadership practices; and sources of power. The report is based on a multisite case study of three schools, serving as an extension of an earlier study of seven secondary schools where the staff were undergoing various change initiatives. For the current study, school personnel identified 12 teacher leaders, whereupon data on these leaders were collected through onsite interviews. The findings suggest that the perceived influence of teacher leaders in the three schools was linked to the context of the school and the conditions as revealed through the eyes of the teachers and principals. School A was small and collegial and the principal worked with the strengths of the teaching staff. At school B, the principal implicitly trusted the teacher leaders and power was shared at the grassroots level with teacher leaders' decision-making mainly occurring at the managerial level. The teacher leaders in school C were able to articulate their concerns to the principal and to colleagues and were able to demonstrate their commitment to providing programs to help improve the students' potential. (Contains 94 references.) (RJM)