Purpose: The authors adapted distributed cognition theory to provide a detailed account of how school leaders use knowledge of the new programs, existing initiatives, and school contexts to guide policy implementation. Research Design: The study used distributed cognition theory to show how policy implementation studies provide an occasion to understand the influence of context on practice. The article focuses on a case study of (a) a suburban district design of a teacher evaluation policy and (b) a principal's effort to use the evaluation program with the teachers in her middle school. The authors adapted the distributed cognition theory to provide an analytic framework to better address the issues of school leadership. Findings: The authors found that the design of the policy required teacher evaluators to address the tensions between summative and formative evaluation implicit in the program design. In this case, the principal relied heavily on her discretion to determine which features of the teacher evaluation policy would be emphasized with different teachers. The case also provided insight into how the principal reconciled the demands of evaluation with ongoing instructional and personnel demands. Conclusions: The distributed cognition framework provided a valuable tool for organizing close studies of the cognitive and contextual dimensions of leadership practice and can provide valuable information about how policies can be designed and used to shape real changes in everyday practice. (Contains 2 tables.)