In many school districts, the pressure to raise test scores has created overnight celebrity status for formative assessment. Its powers to raise student achievement have been touted, however, without attending to the research on which these claims were based. Sociocultural learning theory provides theoretical grounding for understanding how formative assessment works to increase student learning. The articles in this special issue bring us back to underlying first principles by offering separate validity frameworks for evaluating formative assessment (Nichols, Meyers, & Burling) and newly-invented interim assessments (Perie, Marion, & Gong). The article by Heritage, Kim, Vendlinski, and Herman then offers the most important insight of all; that is, formative assessment is of little use if teachers don't know what to do when students are unable to grasp an important concept. While it is true that validity investigations are needed, I argue that the validity research that will tell us the most—about how formative assessment can be used to improve student learning—must be embedded in rich curriculum and must at the same time attempt to foster instructional practices consistent with learning research.