The research is clear: What teachers do in their classrooms matters. But which practices really make a difference? John Hattie (2012) conducted an extensive meta-analysis, looking at 800 meta-analyses that focused on locating a specific student achievement outcome and identifying an influence on that outcome. Formative assessment topped his list of the most influential practices that improve student outcomes. What makes formative assessment so effective? It depends on whom you talk to. Although experts tell us that formative assessment is one of the most powerful ways to raise student achievement (Black & Wiliam, 1998), we don't always know which practices are most effective, when to deploy them, and why a particular combination actually worked for a particular student in a particular classroom. We often hear that the best feedback practices must be specific, addressable, timely, ongoing, and content-rich (Wiggins, 2012). But many beginning teachers and administrators don't have a clear idea of what these terms mean.