Few ideas have impacted the church more than reform, but in recent centuries it virtually disappeared from theological discourse. That changed on December 22, 2005, when Pope Benedict XVI, in his address to the Roman Curia, introduced “hermeneutic of reform” as the proper category for interpreting Vatican II. John O’Malley here traces the history of the idea of reform, describes its meaning in different contexts, and shows how the problem of change is at its very core. He then shows how Vatican II dealt with the problem and concludes with an analysis of Benedict’s address.