Quantitative research measuring teacher leadership's impact in education is lacking, specifically at the site level. It has been proven time and again that most successful schools have well-established and strong leaders with a great capacity to lead. While the world of transformational teacher leadership is intensifying on many levels, it resonates with mainstream educators and evokes great debate and conversation at the local, state, and national levels due to a lack of an agreed upon definition, collective understanding of the significance for the role of teacher leaders, and the generalized belief that all teachers are leaders. Leadership is about direction and influence. Teacher leadership is about establishing a shared vision and direction for the grade level and collective site and developing teacher leaders who influence others during a time of change. This study examined 74 teacher leaders assigned to either the treatment (TLCA) or the comparison group (Reflective Coaches) using the Leadership Practices Inventory (LPI). A Doubly Repeated Measure analysis of the TLCA group was implemented using pre- and post-score analysis for each practice, followed by an analysis of interaction to examine the improvement of scores. A paired t test showed significant difference between pre- and post-LPI scores for each of the leadership practice. An ANCOVA and MANCOVA were conducted. The MANCOVA analysis did not yield significant results; the overall descriptive statistics, the doubly repeated measure and the paired t test, indicated increased teacher leader behavior scores at the 0.05 significance levels (p < 0.05). Therefore, research-based professional development for teacher leaders increases leadership behaviors and capacity in teacher leaders. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]