With states and districts across the country implementing the Common Core State Standards, teachers have more opportunities than ever to collaborate around the shared goals of strengthening curriculum design, classroom practices, and student learning. Building from this premise, the Common Assignment Study (CAS) has brought together teams of teachers from Colorado and Kentucky to design and teach two common assignments each year. The experiences of these teachers show how practitioners in different contexts can successfully collaborate to help students meet college- and career-ready standards. Published first online in April 2016 at http://www.kappancommoncore.org/teachers-collaborate-across-state-lines-to-design-high-quality-units/ Kappan Common Core web site.