What is teacher leadership, and what does it look like in action? Part of a qualitative study to better understand how teachers learn to exercise leadership, this article illustrates indicators of informal teacher leadership through the lived experiences of eight practicing elementary and middle school teachers. Characterizing informal teacher leadership as student-centered, action-oriented, and positively influential on teaching and learning beyond one's own classroom, the article illustrates informal teacher leadership through student advocacy and school-level leadership roles and responsibilities. Motivated by student-focused concerns, teachers in the study positioned themselves for informal teacher leadership intentionally and unintentionally. Through their experiences, some teachers grew professionally and built leadership self-efficacy. The article illustrates that a student-centered, action-oriented approach to teaching establishes a foundation for informal teacher leadership, but positive influence on teaching and learning beyond one's own classroom distinguishes informal teacher leadership from simply teacher professionalism.