What makes school reform on a large scale so difficult? This may be the central question vexing education theorists and policymakers today. Optimistic visions of remaking America's schools have given way to the sober recognition that systemic reform—changing what goes on in classrooms across districts, states, and the country as a whole—is much harder than anyone imagined it would be.

"A significant body of circumstantial evidence points to a deep, systemic incapacity of U.S. schools, and the practitioners who work in them, to develop, incorporate, and extend new ideas about teaching and learning in anything but a small fraction of schools and classrooms," says Richard Elmore of Harvard's Graduate School of Education. "Innovations that require large changes in the core of educational practice seldom penetrate more than a fraction of schools, and seldom last for very long when they do."