This interactive electronic report provides an overview of an innovative new instrument developed by CPRE researchers to authentically measure teachers’ formative assessment practices in mathematics. The Teacher Analysis of Student Knowledge, or TASK, instrument assesses mathematics teachers’ knowledge of formative assessment and learning trajectories, important components of the instructional knowledge necessary to teach to the high expectations of the Common Core State Standards (CCSS). Researchers found that the majority of teachers of mathematics in grades K-10 in urban and urban fringe districts focused on their students' procedural skills rather than their conceptual understandings, indicating that there is significant room for growth in teacher capacity to identify, interpret, and respond to students' conceptual understanding.