The positioning of theory in relation to educational practice has provoked much recent debate, with some arguing that educational theory constrains thinking in education, while others dismiss "theory" out of hand as belonging to the world of the "academic," abstracted from the "realities" of the classroom. This paper views theory as necessarily implicated in all practices, but argues that depending on the theories embraced, and the understanding of theory itself, education can be understood in very different ways. Resisting the separation of theory from practice, the paper takes up the call to consider the entanglement of theory with practice, or how theory matters. It takes formative assessment as a particularly fertile case for this discussion. Formative assessment has been considerably developed in schooling across different national education systems. Its aspiration is for assessment to support learning, rather than only to credentialise learning. Having first emerged as a concept when behaviourism held sway, it has been considered through different theoretical lenses. Drawing upon empirical studies of classroom assessment practices, the paper draws out the different "mattering" implicated in the different languages of assessment used by practitioners, raising questions about the practices this produced. The paper concludes by asking if formative assessment could become "educational" in a more radical sense, if opportunities to focus on the contingencies and politics of our meaning-making were sometimes taken up more openly and dialogically with students, as opposed to formative assessment sitting in a instrumental relationship to a given curriculum.