Understanding Consequential Assessment Systems for Teachers (UCAST) studied a large urban school district in the United States as it adopted a new teacher evaluation system. This project studied 1,000 principals as they were trained to use this new system, and followed them into their first three years of implementation. This article features a multiple-case case study of three of these school principals. Findings indicate principals exercise professional judgment by donning evaluation policy as a "mantle of agency" for working toward a host of aims sometimes aligned with and sometimes antithetical to stated policy. Implications for policy and practice are discussed.