RELEVANCE TO THE CLINICIAN: Clinicians already know that not all patients who are diagnosed with rheumatic diseases really have them. Moreover, determining which patients have improved and by how much is also difficult. Classification criteria allow clinical researchers to recruit patients with similar diseases (e.g., rheumatoid arthritis or systemic lupus erythematosus) into studies. Response criteria help to determine whether treatments really work, i.e., whether they actually produce clinically important improvement. As the science of clinical research advances, we must update our standards for considering classification and response criteria. In this editorial, members of the American College of Rheumatology (ACR) Subcommittee on Classification and Response Criteria describe the purpose of criteria sets, their development and validation, and the role of the ACR in adopting them.