There is an ongoing tension within educational policy worldwide between countries that seek to reduce the opportunities for teachers to exert judgement and control over their own work, and those who seek to promote it. Some see teacher agency as a weakness within the operation of schools and seek to replace it with evidence-based and data-driven approaches, whereas others argue that because of the complexities of situated educational practices, teacher agency is an indispensable element of good and meaningful education. While the ideological debate about the shape and form of teacher professionalism is important, it is equally important to understand the dynamics of teacher agency and the factors that contribute to its promotion and enhancement. In this paper, we draw from a two-year study into teacher agency against the backdrop of large-scale educational reform--the implementation of Scotland's "Curriculum for Excellence"--in order to explore these questions. We focus on teachers' beliefs in order to get a sense of the individual and collective discourses that inform teachers' perceptions, judgements and decision-making and that motivate and drive teachers' action. While the research suggests that beliefs play an important role in teachers' work, an apparent mismatch between teachers' individual beliefs and values and wider institutional discourses and cultures, and a relative lack of a clear and robust professional vision of the purposes of education indicate that the promotion of teacher agency does not just rely on the beliefs that individual teachers bring to their practice, but also requires collective development and consideration.