Background: Instructional coaching has gained popularity as a reform instrument, yet it varies widely across contexts. This variability plays a role in weak implementation or even rejection of coaching within schools. Further, there are gaps in our understanding of how coaching is adopted and accepted in different educational systems. Purpose: This article uses concepts from organizational institutionalism to gauge the legitimacy and taken-for-grantedness of coaching in two charter-management organizations and one public school district. It surfaces the processes as well as the outcomes of the institutionalization of coaching. Research design: I collected qualitative data for this study in three systems to draw out comparisons in the structures, practices, and norms regarding coaching: This included 38 interviews, over 20 observations, and over 30 documents. I coded and analyzed the interview, observation, and document data to answer questions about how and why coaching was institutionalized in each system. Findings: My findings reveal that coaching was more highly institutionalized in the two charter-management organizations than in the public school district. In particular, coaching was deemed appropriate and desirable by most educators in the charter systems. Additionally, coaching was embedded in system-level policies and school-level routines in the charter systems. My findings also indicate that organizational structures, actors' role definitions, and artifacts were associated with the institutionalization of coaching. Conclusions: This study sheds light on how and why coaching, as a counternormative lever for instructional reform, is institutionalized in various educational systems. It points to the importance of system and school leaders' routines for increasing the legitimacy and taken-for-grantedness of coaching.