School leaders and principals have an obligation to use every tool at their disposal to maximize student achievement. All students deserve the best use of data to inform the decision-making of those entrusted to deliver the finest education available to them. The purpose of this study was to ascertain the perceptions of principals in Los Angeles Archdiocesan high schools about the use of assessment data in their schools by finding how they were using assessment data to inform curricular and pedagogical decisions, and then determining what factors affect the use of assessment data to inform their curricular decision-making. This study was a mixed-method investigation using a quantitative survey to find processes in Archdiocesan high schools that capture and utilize assessment data to inform decision-making, as well as to determine the principals' perceptions of the benefits and challenges related to assessment data usage. The qualitative aspect of this study consisted of interviews of Archdiocesan high school principals meant to expand upon the findings of the survey. The findings of the study, viewed through the lens of a conceptual framework, suggest a breakdown in the use of data from the very beginning of the process. Standardized assessment data are the information used to drive curricular decisions while data from formative assessments and curriculum maps, are utilized less frequently. The study also found that, while principals feel that their teachers valued the use of data, there was room for growth in the protocols enlisted to analyze assessment data, and in the cultivation of a culture of collaboration and learning. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]