Education reform requires the commitment and investment of teachers if it is to succeed. Recognising the importance of teacher engagement, some countries have made teacher agency a feature of their curricula. Wales has embraced the notion of teacher agency within the building of its new curriculum by creating a body of Pioneer teachers to shape its new curriculum framework. This paper considers the nature of teacher agency experienced by a group of these Pioneers working on the expressive arts area of the curriculum. It does so through an exploration of the ecological nature of teacher agency, as theorised by Emirbayer and Mische (1998), and it considers agency through a framework of different levels: the micro-level focuses on the individuals and their personal contributions; the macro-level considers Pioneers' work at national level, liaising with teachers from across the country and taking responsibility for creating the curriculum; the meso-level refers to where the two former levels come together, i.e. the Pioneers' work within their own institution, trialling the new curriculum. The evidence indicates that teacher agency was easier to achieve at micro-level and macro-level, than at meso-level. This paper suggests, therefore, that achieving teacher agency at institutional level is more complex and challenging than is the case at the other levels. Greater understanding and attention are, therefore, needed about how to achieve teacher agency in teachers' different spheres of work, particularly when working at institutional level.