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 perspective on language development for secondary-grade English Learners that underpins the CAS Framework and notes its implications for assessment. [For Brief II, see ED626109; Brief III, see ED626110; and Brief IV, see ED626111.]