The author develops the basic idea of evaluation policy, describes a practical model for development and revision of evaluation policies (including a taxonomy, structure, and set of principles), and suggests critical challenges and opportunities for the future of evaluation policy. An evaluation policy is any rule or principle that a group or organization uses to guide its decisions and actions when doing evaluation. Every entity that engages in evaluation, including government agencies, private businesses, and nonprofit organizations, has evaluation policies. Sometimes they are explicit and written; more often they are implicit and ad hoc principles or norms that have simply evolved over time.