This document discusses moving from traditional supply-driven library collections to demand-driven collections using data analysis. It notes that collection budgets are unsustainable under traditional models and that data can help lower costs by making collections more precise and responsive to user needs. The document advocates analyzing usage data to modify collecting practices and asserts that demand-driven, user-focused models will become a larger share of budgets. It provides examples of how North Carolina State University uses data like usage statistics, citations, and user feedback to evaluate resources and make evidence-based decisions about collections.