Following trends across the developed world to devolve power and responsibility for public services to more local agencies, curriculum reforms in several countries have been characterised by policies designed to increase teacher agency and professionalism as a means of achieving successful change. In Wales, this approach has been promoted through adoption of a principle of subsidiarity. Four interconnected reasons are presented for its use within the development of a new curriculum. These can be summarised as follow: that it will encourage teacher professionalism; stimulate improvements in teaching; enhance the responsiveness of schools to local and national needs; lead to increased confidence in the reforms. This paper explores the extent to which these four justifications are reflected in teachers' experiences of curriculum development in schools involved in leading the reform process. Drawing on data from 10 in-depth semi-structured interviews with teachers and a survey of over 600 teachers, data reveal that although the principle of subsidiarity is broadly welcomed, whether or not its application fulfils the justification criteria presented is far from clear. Questions about the application of a subsidiarity principle in relation to curriculum reform are discussed.