Accurate pain assessment is a core nursing responsibility and a frequently tested NCLEX-RN skill because pain is subjective, undertreatment and overtreatment both cause harm, and choosing the right validated tool for the right client is essential to safe care. The guiding principle is that pain is whatever the experiencing person says it is, existing whenever the person says it does; the client's self-report is the single most reliable indicator of pain and the gold standard. When a client can self-report, the nurse uses numeric or descriptive scales; when a client cannot self-report, because of young age, cognitive impairment, sedation, intubation, or the end of life, the nurse selects a validated behavioral observation tool. This lesson covers the major scales every RN must know and match to the population: the Numeric Rating Scale and the Visual Analog Scale for adults who can self-report, the Wong-Baker FACES scale for children and others who respond better to faces, the FLACC scale (face, legs, activity, cry, consolability) for infants and young children or nonverbal clients, the CPOT (Critical-Care Pain Observation Tool) for critically ill...