Beginning in 2009, the United States (US) federal government spearheaded a nationwide teacher evaluation reform effort, encouraging states to change their process of evaluating teachers. School principals, the primary evaluators of teacher performance, act as middle leaders in the sense they must attempt to balance messages from federal, state, and local level policymakers and the teachers they evaluate about how this process will play out in practice. In this paper I ask: (1) In what ways do US school principals act as middle leaders when learning and implementing new teacher evaluation systems?; and (2) In what ways do principals' actions as middle leaders influence how teacher evaluation systems work in schools? Data for analysis comes from nine K-8 public school principals in the US state of Michigan and includes interviews with principals and teachers (n=36), observations of principals interactions with teachers during the evaluation process (n=18), and additional teacher evaluation artefacts. The results of this work suggest principals act as middle leaders by trying to balance messages received from their superiors and the teachers they evaluate. Implications for policy and practice are discussed.