Curriculum reforms often fail because teachers are asked to implement approaches to teaching and learning that are contrary to their beliefs about students and how to teach them. Since curriculum reforms are attempts to resolve particular educational problems, failures in implementation produce parallel failures to resolve the problem for which the curriculum was the intended solution. We propose an approach to understanding curricula that makes the mismatch between assumptions about teaching and learning in policy and teachers' practice apparent. By describing curricula as theories of action we can explain why teachers persist in teaching as they usually do. Once described, the gap between policy and implementation can be bridged by creatively redesigning the current theories that underpin them. We demonstrate this approach to understanding curricula by describing the espoused and enacted locally designed curriculum of a Japanese junior and senior high school programme and find that teachers' strongly held beliefs about their students constrained them from enacting the active student-centred approaches they had initially espoused. In so doing, we show how even where there is broad agreement between teachers and policy makers there may be a considerable gap between the curriculum that was intended and that which is enacted in the classroom. By deeply understanding and comparing the assumptions underpinning both curricula, we explain such mismatches and discuss how they might be reduced through creative and principled reformulation.