Recent innovations in teacher evaluation include the development of standards-based frameworks for observational evaluations of teacher practice and development of test-based measures of teacher effectiveness. These tools raise questions about how best to identify good teaching, about the roles of both evaluations and principals in improving teaching, and about the relationship between school context and teacher evaluation. This qualitative study examines principals' understanding and use of a test-based ranking of teacher quality--the Academic Growth over Time (AGT) ranking--and a standards-based observation framework--the Teaching and Learning Framework (TLF)--in the Los Angeles Unified School District. Findings suggest principals believed outcomes matter, but harbored concerns about what AGT rankings actually measured. In addition, principals felt AGT rankings provided little information about how to improve teaching. Some were concerned that in low-scoring schools, the AGT rankings might reinforce test-focused instruction in ways that disadvantaged students. Principals preferred the TLF because it was more comprehensive than previous observation tools, helped teachers evaluate their practice against clearly defined expectations, and helped principals improve instruction. Though principals used the TLF to assign teachers ratings, they primarily described it as a tool to improve the quality of teaching in their buildings. Principals noted that the TLF process placed heavy demands on the time of teachers and principals, and expressed concern about how they could sustain high-quality implementation of the TLF when they had to use it at scale. Most principals noted cases where the AGT rankings indicated a different reality about a teacher's effectiveness than what the principals expected, based on classroom observations. How principals reconciled discrepancies varied. Principals in higher-scoring schools were more likely to discount test-based measures and more likely to emphasize the professional capability of their staff as an asset. The findings suggest principals' use of evaluation tools is mediated by their confidence in the tools and their own leadership, by how useful they feel the tools are, and by their own school contexts. In addition, this study suggests that the unique burdens of persistent socioeconomic and racial segregation may hamper local efforts to implement promising practices at some sites. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]