The Comprehensive Assessment System (CAS) Framework presents a vision for a system of assessments for English Learners in secondary grades that brings assessment closer to the classroom and fully involves teachers in assessment development and validation. The CAS Framework is intended to signal a new and equitable direction and to provoke reflection and debate among all those concerned with improving outcomes for English Learners. This brief describes the approach to validating assessments included in the CAS. [For Brief I, see ED626108; Brief II, see ED626109; and Brief III, see ED626110.]