Counter to narratives of persistently failed school reform, we argue that reforms sometimes succeed and seek to understand why. Drawing on examples from the founding of public schools to the present, we find that successful system-wide reforms addressed problems that teachers thought they had by being consistent with prevailing norms and values, mobilizing a significant public constituency, and building the needed educational infrastructure. We distinguish between system-wide and niche reforms, suggesting that some--particularly those seeking ambitious instruction--failed system-wide but succeeded by creating protected educational niches. We conclude with a discussion of the implications for the Common Core.