This article describes the role of language and literacy assessment in Response to Intervention, the assessment requirements and intent of RTI legislation, and the assessment principle in the International Reading Association's guiding principles on RTI. It outlines the multiple purposes of assessment required for a coherent RTI system, including screening, diagnostics, formative progress monitoring, benchmark progress monitoring, and summative outcome assessment. Building on this framework, the article then presents types of knowledge and skills teachers and specialists need in order to use assessments and their results effectively in an RTI approach to preventing language and literacy problems.