The Impact of Teacher Leaders: Case Studies from the Field is designed to celebrate, spotlight, teach, inspire and encourage. First and foremost, the authors wish to celebrate the accomplishments and dedication of teacher leaders. Research clearly shows that teachers have the greatest impact on student achievement. It is also evident that teacher leaders are the bridge between their colleagues in the classrooms, and administrators and parents. Initiatives can be driven, impaired or abandoned based on the support and involvement from teacher leaders.  Teacher leadership is critical in all schools. For this reason, the authors spotlighted cases from coast to coast in the United States, along with some international examples. In an era when teacher preparation programs have decreased in numbers, statistics are showing that those who successfully complete those programs are not remaining in the field, and veteran teachers are leaving in record numbers, it is important to spotlight the success of teachers through their leadership roles in very different school contexts. Case studies are an important tool in qualitative research as they provide examples of implementation; examples of relationships between theory and practice; allow close examination of people and contextual situations; allow data collection and analysis, and allows for further study of people, groups, events, reasons and questions. Case studies utilize a variety of sources and use different methods. For the purposes of this companion publication, purposeful selection spotlighted 24 case studies where the authors interviewed teachers to collect and write narratives about their teacher leadership roles, often focusing on a specific leadership experience. All participating teachers were under a teacher contract at the time of the teacher leadership case study experience and were leading at the building or district level.