In 2012, the Board of Governors of the California Community Colleges adopted recommendations from the legislatively mandated Student Success Task Force. They included a recommendation that California's community colleges develop "diagnostic" assessments for use across the system. These would provide detailed information about each student's specific academic strengths and weaknesses. If implemented, they would be a component of new assessments that all community colleges in the state would use to place under-prepared students into "developmental" English and math courses below the college level. As California takes on this challenge, it can learn from the experiences of several states that have already developed, or are developing, statewide diagnostic tests for their community colleges. These states' reforms are still in their infancy, and how they will play out over the long term is not yet clear. But their efforts help clarify the crucial issues for California to consider and how the ambitious reforms envisioned by the Task Force might differ from those being undertaken in other states. This report: (1) Introduces "diagnostic assessment" and why it has attracted attention as a tool for community college reform; (2) Describes how several community college systems--Florida, Virginia, and North Carolina--are developing their own diagnostic assessments; (3) Shows that California's proposed reforms differ because statewide diagnostic assessments would be developed without prior agreement about how the developmental curriculum should be structured; (4) Discusses practical implications related to the time needed for testing and the resources needed to use diagnostic information effectively; and (5) Describes the opportunity that California colleges and K-12 schools have to better coordinate their expectations and assessments for students. Fundamental to this discussion will be the K-12 system's recent adoption of Common Core State Standards.