This study aims to investigate the factors affecting teacher leadership in high schools. This study was designed as a qualitative case study and a total of 21 teachers employed in high schools in Karabuk province participated in the study. Participants of the current study were chosen through maximum variation, convenient and criterion sampling which are among purposeful sampling methods. A semi-structured interview form developed by the researchers depending on both the related literature and expert opinions was used to gather data. Content analysis was performed to analyze the data. The findings of the study were gathered under four themes entitled the purpose of teacher leadership, the factors that impact teacher leadership, the problems encountered by teacher leaders and the proliferation of teacher leadership. Results revealed that teachers reported supporting each student's learning, improving teaching in the school, and building an effective school culture have been among the primary purposes of teacher leadership. According to the results principal leadership, school climate, and school culture are the leading factors that affect teacher leadership. Results also revealed that teacher leaders encountered problems depending on insufficient support of school administration for teacher leadership, egalitarian norms among colleagues and exam-oriented education system and that they thought teachers' career opportunities should be expanded, an adequate increase in their salaries should be supported and practices ensuring teacher autonomy should be performed in order to expand teacher leadership practices.