The contemporary interest in researching student agency in science education reflects concerns about the relevance of schooling and a shift in science education towards understanding learning in science as a complex social activity. The purpose of this article is to identify problems confronting the science education community in the development of this new research agenda and to argue that there is a need for research in science education that attends to agency as a social practice. Despite increasing interest in student agency in educational research, the term "agency" has lacked explicit operationalisation and, across the varied approaches, such as critical ethnography, ethnographies of communication, discourse analysis and symbolic interactionism, there has been a lack of coherence in its research usage. There has also been argument concerning the validity of the use of the term "agency" in science education research. This article attempts to structure the variety of definitions of "student agency" in science education research, identifies problems in the research related to assigning intentionality to research participants and argues that agency is a kind of discursive practice. The article also draws attention to the need for researchers to be explicit in the assumptions they rely upon in their interpretations of social worlds. Drawing upon the discursive turn in the social sciences, a definition of agency is provided, that accommodates the discursive practices of both individuals and the various functional social groups from whose activities classroom practice is constituted. The article contributes to building a focused research agenda concerned with understanding and promoting student agency in science.