Classification systems help orthopaedic surgeons characterize a problem, suggest a potential prognosis, and offer guidance in determining the optimal treatment method for a particular condition. Classification systems also play a key role in the reporting of clinical and epidemiologic data, allowing uniform comparison and documentation of like conditions. A useful classification system is reliable and valid. Although the measurement of validity is often difficult and sometimes impractical, reliability-as summarized by intraobserver and interobserver reliability-is easy to measure and should serve as a minimum standard for validation. Reliability is measured by the kappa value, which distinguishes true agreement of various observations from agreement due to chance alone. Some commonly used classifications of musculoskeletal conditions have not proved to be reliable when critically evaluated.