Teacher leadership is a growing reform in the United States, with almost 300 active programs that are preparing, positioning, and/or rewarding teacher leaders (Berg et al., 2019). Despite this plethora of activity, there is relatively little empirical research on the influence of teacher leadership on teacher practice. This study adds to the research base on how teacher leaders influence instruction. The study focuses on the instructional influence of teacher leaders in formal teacher leadership programs in four districts in three American states. Although the programs had different emphases and structures, all four provided training, ongoing support, and formal school positions for teacher leaders, who were charged with working with teachers to support instructional improvement. Using survey data from approximately 1,050 teachers in 45 schools in the four districts and interview data from a subset of schools, we examined the relationship between the activities of teacher leaders and teacher reports of teacher leader influence and changes in instruction. The findings indicate that teachers who report being influenced by their teacher leader also report more frequently planning with their teacher leader, engaging in professional learning activities with their teacher leader, and participating in observations and feedback with their teacher leader. Additionally, higher levels of teacher leader influence are associated with teacher reports of change in instructional practice.