This paper discusses practical implications of formative assessment effectiveness research in grades one through six. Formative assessment effectiveness is a timely topic for state and district leaders as they develop implementation plans for Every Student Succeeds Act (ESSA), including their approaches to supporting school-based practitioners (Shepard, Penuel, & Davidson, 2017). This paper addresses one overarching research question: What effectiveness claims can we responsibly make about formative assessment? To improve on prior research, we adopted Bennett's (2011) recommendations for conducting research on the effectiveness of formative assessment, including: precisely defining formative assessment, considering whether studies of the impact of formative assessment were conducted with sufficient quality to support causal inferences, and describing concrete instantiations of effective formative assessment.