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. Evaluation policies profoundly affect the day-to-day work of all evaluators and ultimately the quality of the programs they evaluate. Many recent and current controversies or conflicts in the field of evaluation can be viewed, at least in part, as a struggle around evaluation policy. Because evaluation policies typically apply across multiple evaluations, influencing policies directly may have systemic and far-reaching effects for practice. Evaluation practice must play a critical role in informing and shaping the development of evaluation policies. (Contains 5 figures and 1 note.)