Research shows that professional development alone does not provide adequate leadership preparation for teachers, yet many develop into established teacher leaders. The purpose of this study was to explore how teachers learn to exercise informal leadership in the schools and districts where they work. Eight elementary teachers who lived and worked in the Midwestern region of the United States participated in the study. Data collection included written self-reflections, questionnaire responses and a participant focus group. Teachers in the study learned to exercise leadership gradually, over time as they accumulated professional experience. Findings of the study revealed three factors that developed teacher leadership: exposure to research-based practices, increased teacher self-efficacy, and serving beyond the classroom. Professional development designed to improve teaching practice, paired with job-embedded collaboration, supported teachers' progress toward leadership. Contrary to the findings of similar studies, leadership-focused professional development provided only peripheral support. Taking this research into account, the study concludes that professional development designed to improve teaching practice and build leadership skills, paired with job-embedded collaboration, supports teachers toward leadership gradually, over time, as they accumulate professional experience and increase self-efficacy. (Contains 1 table and 1 figure.)