Assessments are a way for stakeholders in education to understand what students know and can do. They can take many forms, including but not limited to paper and pencil or computer-adaptive formats. However, assessments do not have to be tests in the traditional sense at all; rather, they can be carried out through teacher observations of students or portfolios of students' work. Regardless of form, when assessments are well designed and a component of a system of teaching and learning that includes high-quality instruction and materials, they are part of the solution and not a source of the problem. Thus, debates on whether or not to assess students fail to create a worthwhile discussion about testing in schools and how to make assessments better. This report presents a theory of action that assessments should play in schools. This report is part one in a series about the future of testing in America's schools. Part two "Artificial Intelligence. Future of Testing in Education" (ED617052) reviews advancements in technology, with a focus on artificial intelligence that can powerfully drive learning in real time. Part three "The Way Forward for State Standardized Tests. Future of Testing in Education" (ED617055) looks at assessment designs that can improve large-scale standardized tests.