For many years state education agencies (SEAs) have sought to create assessment systems that include all students, including students with disabilities. In order to improve outcomes for students with disabilities, there is an urgent need for cross-agency collaboration between special education and assessment offices, but other offices (e.g., curriculum and instruction, accountability) may also need to be involved. The purpose of this brief is to shine a light on various aspects of cross-agency collaboration with a goal of helping SEAs initiate or improve their collaborative processes. The reasons for collaboration and its benefits are discussed, suggestions for who needs to be involved are provided, along with specific examples of successful collaboration.