During the past several years, one of the most frequently addressed topics at conferences and in professional journals is responsiveness to intervention (RTI). This is because it is viewed by many as a new way to think about both early intervention and disability identification. Despite its relative newness, a subset of RTI proponents contend researchers and practitioners know everything they need to know to implement it effectively. We disagree and argue that the smartest and most responsible way to move forward with RTI implementation is to recognize what all of us collectively do know and do not know. In this article, we identify unresolved issues--general and specific--important to RTI implementation, teacher effectiveness, and student achievement.