Student achievement is central to the policy debate over charter schools. This paper reviews what is currently known about charter schools' impact on student achievement. After documenting the dearth of systematic empirical studies on the topic, the paper combines the findings of existing research into an overall impact rating, weighted by the studies' methodological quality. It finds that the existing body of research reveals a mixed picture, with studies from some states suggesting a positive impact, studies from other states suggesting a negative impact, and some providing evidence of positive and negative impacts. Overall, the charter impact on student achievement appears to be mixed or very slightly positive. However, this conclusion is tempered by the fact that there are, as yet, no systematic studies of charter achievement in several states that have large numbers of charter schools. The paper concludes by offering some preliminary explanations for variations in charter performance across states and for the paucity of empirical evidence in many states. (Contains 48 references.) (Author/SM)