Although research in science education has led to new assessment forms and functions, the reality is that little work has been done to unpack and capture what it means for a teacher to develop expertise at assessing science. The purpose of this paper is two-fold. First, I suggest a conceptualization of assessment expertise that is organized around three dimensions: (a) designing aligned and theoretically cohesive assessment (Design), (b) using assessment to support students' science learning (Use), and (c) equitably assessing language minorities (Equity). The second purpose is to suggest and exemplify various levels of teaching expertise across the three conceptual dimensions using written assessment plans gathered from a study on secondary science pre-service teachers' assessment growth. The contribution of this paper lies in its further conceptual development of assessment expertise, instantiated in a rubric, which can spark discussion about how to capture the range of assessment practices that might be found in science classrooms as well as move toward a potential learning progression of assessment expertise. (Contains 2 tables.)