Purpose: In the last 13 years, schools across the country spent a significant amount of time, money, and energy transitioning to the Common Core State Standards. States have also adopted high stakes accountability systems to track the achievement and growth of students. Despite these efforts, most schools are demonstrating only modest growth in the area of mathematics. This study examines the instructional leadership behaviors of elementary school principals of schools with above-average growth in student mathematics achievement. Conceptual Framework: The Hallinger and Murphy instructional framework is divided into three main areas: defining the school's mission, managing the instructional program, and promoting a positive school environment. The framework is the most commonly used instructional leadership framework in the literature. Methodology: This qualitative case study design used interviews and artifacts to answer the central research question: What are the instructional leadership actions of principals in elementary schools with increasing student achievement in mathematics? The researcher interviewed elementary school principals of schools that improved student achievement in mathematics on the Smarter Balanced summative assessment. The interview was semi-structured with open-ended questions. Findings: Based on the three research questions, 12 themes were identified in this study. Some of the study's findings include the schools developed clear goals for improving student achievement, principals relied heavily on data to monitor student progress and drive instructional changes in classrooms, and that principals supported their teachers with high-quality professional development on mathematics by leveraging district resources and math-specific professional development. Conclusions: The study concluded that highly effective elementary school principals engaged in all three of the main dimensions of instructional leadership. The study also concluded that schools can significantly increase student achievement in math regardless of their initial level of student achievement and regardless of the school demographics. Recommendations: It is recommended that principals develop clear goals for their school around mathematics and then regularly review student achievement data to guide instructional practices and identify students who need a math intervention program. Principals should also promote high-quality professional development based on effective teaching practices. Finally, school districts should support principals in these efforts and becoming better instructional 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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]