School districts increasingly see interim assessments as an important element in their instructional improvement strategy and are implementing interim assessments district-wide in multiple grades and content areas. Yet, they face significant design and implementation challenges to realizing the potential of interim assessments to improve teaching and learning. The Aspen Institute Education and Society Program and Achieve partnered with the National Center for the Improvement of Educational Assessment to develop this policy brief. The goals are to clarify how interim assessments fit into the landscape of formative assessment and to offer recommendations to districts on the appropriate use of interim assessments as part of a broader assessment system. The brief is informed by conversations with the Aspen Institute Urban Superintendents Network and the affiliated Dana Center-Achieve Urban Mathematics Leaders Network.