How can good educational practice move beyond pockets of excellence to reach a much greater proportion of students and educators? While many children and young adults in school districts and communities around the country have long benefited from the tremendous accomplishments of successful teachers, schools, and programs, replicating this success on a larger scale has proven to be a difficult and vexing issue. In this article, Richard Elmore addresses this problem by analyzing the role of school organization and incentive structures in thwarting large-scale adoption of innovative practices close to the "core" of educational practice. Elmore then reviews evidence from two attempts at large-scale reform in the past--the progressive movement and the National Science Foundation curriculum reform projects--to evaluate his claims that ambitious large-scale school reform efforts, under current conditions, will be ineffective and transient. He concludes with four detailed recommendations for addressing the issue of scale in improving practice in education.