The overall architecture and major components of a measurement system designed and evaluated to support Response to Intervention (RTI) in the areas of language and literacy in early childhood programs are described. Efficient and reliable measurement is essential for implementing any viable RTI system, and implementing such a system in early childhood programs presents several special challenges. This article describes (a) the general approach taken to developing and evaluating this measurement system, including its conceptual and methodological foundations, methods, and results for developing items and scales for second-generation Individual Growth and Development Indicators (IGDIs 2.0), and (b) use of these scales in a larger decision-making framework for differentiating language and literacy instruction to match the needs of preschool children at risk of academic difficulties. The authors conclude with a review of some challenges associated with developing measurement for differentiated instruction in early childhood settings. (author abstract)