Attempts to scale up instructional interventions confront implementation challenges that mitigate their ultimate impact on teaching and learning. In this article, we argue that learning about adaptation during the design and implementation phases of reform is critical to the development of interventions that can be implemented with integrity at scale. Through analysis of data generated during a mathematics instructional coaching initiative, we examine the adaptations coaches made to diverse relational and organizational contexts. Findings from two studies of adaptation illustrate the need to attend to the extent to which adaptations are consistent with the core features of a reform. Based on our findings, we posit a generalizable model that supports evidence-based mutual adaptation. [For the corresponding grantee submission, see ED599340.]