The role of principal supervisors has changed significantly, moving away from a focus on compliance and towards a greater emphasis on nurturing principals' instructional leadership skills. As a result of this evolution, there is now an increased presence of supervisors in schools, but there are no clear guidelines for their activities. This study aims to fill this gap by developing a theory that explains how principal supervisors can effectively support principals in their roles. The insights gained from this study will have significant implications for the future training and professional development of principal supervisors, which aims to improve the effectiveness of principal leadership and, in turn, enhance student outcomes. The study seeks to answer the key research question: "How do principal supervisors and their principals perceive how principal supervisors provide effective support?" Using a qualitative approach and grounded theory design, the study collected data through 15 semi-structured interviews involving three principal supervisors and 12 principals who had worked together for at least three years. The resulting grounded theory outlines a comprehensive framework that explains how principal supervisors can offer impactful support to principals. Central to this theory is the use of an equity lens by principal supervisors, alongside the strategic use of six key professional tools: time, school visits, data, coaching, collaboration, and professional learning. These tools are enhanced by the supervisors' experience and adeptness in relationship-building. The principal supervisors' support is centered on learning that engages both the principal supervisor and principals, improving the latter's professional practice. This study highlights the importance of continuous learning within the support for both principal supervisors and principals, which facilitates iterative improvement in the principals' professional practice. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]