We raise concerns about the current state of research and development in formative assessment, specifically to argue that in its concentration on "strategies for the teacher", the literature overlooks the "disciplinary substance" of what teachers and students assess. Our argument requires analysis of specific instances in the literature, and so we have selected four prominent publications for consideration as examples. These, we show, pay little attention to the student reasoning they depict, presume traditional notions of "content" as correct information, and treat assessment as distinct from other activities of learning and teaching, even when they claim the contrary. We then offer an alternative image of formative assessment centered on attention to disciplinary substance, which we illustrate with an example from a high school biology class. Assessment, we contend, should be understood and presented as genuine engagement with ideas, continuous with the disciplinary practices science teaching should be working to cultivate. (Contains 1 figure and 9 notes.)