Societal, structural, value-based or economic changes and changes related to technological developments necessitate a continuous development process in the field of education. In responding to the changes, teacher agency becomes a key factor. This study explores when the context of reform provides the basis for the growth of agency and when it disables the potential for teacher agency. The analysis is based on empirical data gathered as part of a large-scale mixed-methods study of the professionalism of Estonian teachers. This study identified the strong presence of professional knowledge and skills among teachers related mainly to classroom work and we argue that this is a result of focusing on short-term professional aims. But a prerequisite for the growth of teacher agency is support so that teachers can develop their professional vocabulary and theoretical knowledge base.