Formative assessment has been widely promoted as a means to support student learning and motivation. This practice has potential for communicating to students the value of what they are learning, both in the classroom and beyond (Brophy, 2008). To make good on those promises, however, requires an understanding of the connections between formative assessment practices and the realities of the social systems in which they are employed. In this article, I consider the importance of classroom systems, including the purposes of assessments, their function in the classroom social structure (including issues of power, identity, and competing values), and teachers' multiple concerns with effects on student learning and motivation. I also address the role of conflicts between the practice of formative assessment and grading policies, departmental assessment norms, school district structures, and external assessment mandates. Finally, the recommendation of Black, Harrison, Lee, Marshall, and Wiliam (2004) are examined in light of these issues. (Contains 1 note.)