Many stakeholders have long argued that teachers should use data from their own teaching as a way to improve their performance. The first chapter of this dissertation is a summary of the literature that advises mathematics teachers about what data they should gather and how they should use those data to improve instruction. The literature is categorized according to three distinct approaches: the diagnostic approach, the methods approach, and the teacher approach. Within the diagnostic approach, teachers collect data to determine which students need to learn what content. Within the methods approach teachers collect data to determine what aspects of teaching contribute to student learning. Within the teacher approach, teachers collect data to help them develop beliefs and knowledge that support effective teaching. The benefits and drawbacks of each of these approaches are identified, and some general guidelines and cautions concerning the use of data to improve instruction are discussed. The second chapter reports on a study of high school mathematics teachers engaged in inquiry to improve their own teaching. A range of stakeholders has argued that for teachers to improve their practice through inquiry, they would have to engage in cause-effect reasoning that connects teaching and learning. When asked to create goals for their teaching, and to gather evidence that could help them evaluate their progress towards these goals, the teachers in this study did engage in cause-effect reasoning. However, the nature of this reasoning was complex, varied and not readily apparent. Furthermore, not all instances appear to have the same potential to improve instruction. Three different types of cause-effect reasoning are proposed and documented, and implications for teachers, professional developers and researchers are discussed. [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.]