In the United States, health care costs are spiraling upward at an unsustainable rate. Concurrently, medical specialties, legislatures, and the population each recognize the need to prove that current medical practices are effective, measured by outcomes. These forces necessitate a change in radiology's role, from emphasizing interpretation and reporting to maintaining and demonstrating quality and value. An important part of this task involves establishing the appropriateness of imaging tests by application of objective data and expert opinion in an effort to optimize resource utilization and modality selection. We explore the justifications for establishing appropriateness use criteria, various paradigms that have been applied to their creation, inherent limitations of appropriateness rules based on imperfect data, and challenges associated with their widespread implementation and utilization. An appendix of key terms that may be unfamiliar to radiologists is included for future reference.