Instructional leadership has been identified as having a significant impact on student success. A constructivist approach to grounded theory was utilized in this qualitative study to explore the question: How is instructional leadership understood and practiced by principals in high schools? A grounded theory of instructional leadership emerged as a result of examining the understandings and perceptions of three high school principals in a large urban school jurisdiction through four sources of data: an online demographic questionnaire, individual interviews, a member checking interview, and school based documents. Through grounded theory analysis five components of instructional leadership practice were identified: leading from inquiry, setting the learning agenda, symbolizing education, extending ownership, and understanding people. These five components contribute to and stem from the core component of the principal as the point of influence. The emerging theory suggests that the intentional employment of these components of instructional leadership practice enhances principal focus on supporting high school student success. This study contributes to the professional knowledge and understanding of instructional leadership practices of urban high school principals in supporting student success and provides guidance for the professional learning of principals in their quest to lead this work within high schools.