Principals are expected to use data to develop a clear vision, stimulate meaningful dialogue among stakeholders, create interventions for struggling students, and improve school programs. The purpose of this three-phase, sequential, mixed-methods study was to examine how principals use summative and formative assessment data to improve academic achievement. This study was grounded in the pragmatic worldview and was concerned with applications and solutions to problems that face educational leaders today. The research examined the level of preparation of high school principals to use assessment data, the value they place on both summative and formative assessment data, the ways the principals use assessment data to improve achievement, and the supports and barriers to effective data use by school principals. The analysis of survey data from 38 high school principals and 10 semi-structured principal interviews yielded the following findings: (a) high school principals felt prepared to use both summative and formative assessment data to improve academic achievement; (b) principals value formative assessments over summative assessments in areas of curriculum and instruction as an indicator of student learning and to gauge overall school success; (c) principals indicated that they use summative more than formative data to complete tasks related to their jobs; and (d) principals rely on their own knowledge as the biggest support to data use and see time as the most significant factor when building a data-driven school culture. Overall, the research found that Orange County principals embrace their roles as instructional leaders and are building collaborative, data-driven school cultures. [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.]