Formative assessment is a process used by teachers and students during instruction that provides feedback to adjust ongoing teaching and learning to improve students' achievements of intended instructional outcomes. Formative assessment means assessment embedded in instruction. That definition was adopted in 2006 by the Council of Chief State School Officers' (CCSSO) Formative Assessment for Students and Teachers State Collaborative on Assessment and Student Standards. Tasks are both instructional and informational; children learn mathematics and teachers have data and a framework that helps them continually notice and gather knowledge about their students' thinking. Formative assessment blends seamlessly into instruction, guiding teaching without stealing instructional time. In this article, the authors focus on only one component of formative assessment--the instructional task--and how teachers can design and choose tasks.