The purpose of this study was to determine the impact of the professional development training provided by the National Institute for School Leadership (NISL) on leadership behavior in schools. In order to achieve this overall objective, the following research questions framed this mixed methods study: 1. To what degree did the National Institute for School Leadership (NISL) training impact school leaders' practices of the five areas of leadership behavior (Model the Way, Inspire A Shared Vision, Challenge the Process, Enable Others to Act, and Encourage the Heart) identified on the Leadership Practices Inventory (LPI)? 2. What demographic variables influence the impact of NISL training on the school leaders in the five areas of the LPI? 3. What change in behavior did the NISL trainees perceive they had maintained in the five areas of leadership practices identified on the LPI six months after completing the training? 4. How did the NISL training change leadership practices of those school leaders who participated? The research methodology utilized the online Leadership Practices Inventory (LPI) before and after the NISL training to provide the quantitative data. Demographic information was analyzed to see if any variables influenced the change in scores on the LPI in the five areas of leadership practice. Those areas include: Model the Way, Inspire A Shared Vision, Challenge the Process, Enable Others to Act, and Encourage the Heart. Then qualitative data was collected through focus groups to determine if there were common themes or patterns regarding the five leadership practices of the LPI. This study utilized an explanatory sequential design, where the quantitative data was collected first, then qualitative data was collected to gain additional information to strengthen the study. The implications of this research study may help schools and school districts predict the potential success a school leaders' behavior can have on the success of the school or school district. The review of literature indicated leadership behavior influences student achievement, climate, culture, teacher efficacy, and instruction. This study will evaluate the impact NISL training of the Executive Development Program has on leadership practices of school leaders of west-central Illinois, which could possibly be a model for other areas in Illinois and the United States. [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.]