Few things can be more polarizing than basing teachers’ evaluations on their students’ achievement. It sounds logical: Teachers should be accountable for what their students have – or have not – learned. But incorporating achievement data into teacher ratings is complicated, especially in the early grades of elementary schools when students typically do not take state standardized tests. With nudges from the federal government through programs like Race to the Top and flexibility waivers from No Child Left Behind, nearly every state is revamping its teacher evaluation system to include student achievement data as a significant factor of a teacher’s rating. Across the country, experiments abound as states and school districts struggle to find sound approaches to measure young students’ achievement for the purposes of teacher evaluation.