Recent curriculum policy in the UK and elsewhere has defined teachers as ‘agents of change’. This [re]turn to teacher agency, heralded in policies such as Scotland’s Curriculum for Excellence, represents a change in the professional orientation of teachers. It not only gives explicit permission to teachers to exert high[er] degrees of professional agency within the contexts in which they work, but actually sees agency as an important dimension of teachers' professionalism. In this paper, I outline an ecological theory of agency – a temporal-relational approach that allows for the analytical separation of cultural, structural, material and individual factors that shape agency. I draw upon findings from the Teacher Agency and Curriculum Change project, a one year ethnographic study within an education authority in Scotland. I discuss empirical findings in relation to two indicative themes, which are are illustrative of the complexity of teacher agency. The first concerns the beliefs and aspirations espoused by teachers implementing the curriculum; the second focuses on the professional relationships experienced by teachers in their working environments. I suggest that current policy is often too narrowly focused on raising the individual capacity of teachers, neglecting cultural and structural factors that impact upon agency.