While outstanding teachers are any school system's most important investment, assessing the quality of instructional practice has proven to be an ongoing challenge for the profession. Despite assertions that effective teachers are the single most important school-related factor responsible for increased learning, no teacher's employment is dependent on their performance in the classroom or the quality of instruction that they provide. This problem has fueled a growing mistrust in school districts nationwide, specifically in the area of teacher evaluation. One possible explanation is that the profession lacks the scaled level of expertise needed to evaluate instruction consistently and in a manner that effectively informs the improvement process. In an effort to both strengthen the teacher evaluation process and significantly improve the quality of instruction in classrooms, an observation instrument was developed which measures the critical skills associated with highly effective teaching. These include a teacher's content knowledge, pedagogical expertise, and the ability to establish a classroom culture conducive to sustained learning. The instrument is also consistent with the new Common Core State Standards, and defines quality as the level at which a teacher facilitates multi-directional interactions with the class that result in authentic cognitive engagement and increased subject matter competence. An important distinguishing factor is the instrument's focus on student outcomes related to participation, critical thinking, and academic language as opposed to traditional observations, which focus on teacher behaviors. The study had both quantitative and qualitative components. Multilevel modeling techniques were used to examine the effects of instructional quality on student growth. trajectories in English and math in two California middle schools. The effects proved both positive and significant in both subject areas, but particularly in mathematics where one standard deviation of instructional quality produced an 11-point gain on the California Standards Test. A cross-case narrative analysis also identified the actions taken by teachers that resulted in the highest and lowest levels of instructional quality. Contributions of this study include an efficient model for evaluating instructional effectiveness, methods for informing and differentiating professional development, and an increased understanding of whether or not all students have access to high quality instruction. [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.]