Does Comprehensive School Reform (CSR) work? Research results have been mixed. Some studies have measured a modest improvement in student achievement; others have found no effect. A team of RAND researchers has approached the question of CSR's effectiveness by first focusing on an even more basic question: Has CSR been implemented? A shortcoming of nearly all previous studies is that they have assumed that schools have implemented their CSR model in its entirety. But what if most schools have implemented CSR only partially or not at all? In such cases, improved student achievement cannot be expected--or, if it does occur, cannot be attributed to CSR. Until the level of implementation can be measured, it cannot be determined whether CSR works, or whether one CSR model works better than another. If CSR is more fully implemented, a key area for future investigation will be to determine whether CSR is effective and, specifically, which CSR practices are the most effective. It will then be possible to determine if CSR is a useful tool in meeting the proficiency goals of the No Child Left Behind Act. [This research brief was produced by RAND Education. It describes work done for RAND Education documented in "Evaluating Comprehensive School Reform Models at Scale: Focus on Implementation" (ED494670).]