The intention of this qualitative study was to explore the effects that instructional data systems have on a building level leader's capacity to evaluate student growth and inform principal practice. Principal leadership practices were examined through the lens of Kouzes and Postners' five practices of exemplary leadership (Kouzes & Posner, 2012). In 2009, President Obama signed the American Recovery and Reinvestment Act (ARRA), which included $435 billion dollars of competitive grant funding called the Race to the Top Fund (RTTT). One of the four-core areas of RTTT is, "Building of data systems that measure student growth and success, and inform teachers and principals about how they can improve instruction" (USDOE, 2009). New York State received 700 million dollars of funding in the second phase of the RTTT program (USDOE, 2013). This study has direct implication to the practice of principals. Research that informs a principal's practice about how to utilize the collection, analysis and interpretations of data more effectively and efficiently into actionable instructional steps will make an important contribution to the field. The sample consisted of 18 principals from six school districts in upstate New York. Larger school districts, with student enrollments above 2000 were selected to ensure that there were multiple building principals available from any particular district that was selected to participate. The study found that a majority of the principals reported that the instructional data systems in use in their districts provided them with the capacity to effectively evaluate student growth. Several different types of instructional data systems were found to be in use in the districts studied. They included assessment systems, student information systems and data management systems. Fifty percent of interviewed principals felt that they have received adequate training on the use of the instructional data systems in their district. There did not appear to be a clear relationship between the data obtained from the instructional data systems and the design of professional development for principals. While there was some indication that the sharing of best practices for the use of instructional data systems and student data among principals existed in the districts, no participant reported that a consistent and well established process for doing so was in use anywhere. Interviews from the study revealed several different delivery models for professional development that the principals received on the data systems they use, however there was no indication that any of the models contained an ongoing component to them. Only four of the eighteen principals interviewed indicated that they were affiliated with a professional organization that was either solely or partially focused on the use of data in schools. Modeling the Way emerged as the number one leadership practice from the model by Kouzes and Posner (2012) that principals employed to inspire teachers to use instructional data systems to inform their practice in the study. The study also explored any potential barriers that may be perceived or exist that might work against a principal's objective to use data effectively in their buildings. Twelve of the eighteen principals indicated that time was one of the two most cited barriers to the effective use of instructional data in their schools. The main conclusion of the study is that principals do use data systems to evaluate student growth and inform their practice. More work remains however in providing principals with meaningful ongoing professional development, strategies to improve best practices, pedagogical data literacy and the reduction of the barriers to implementation identified in the study. [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.]