Increases in payment for more-complicated patients can incentivize upcoding, a practice in which hospitals code more secondary diagnoses or complications to classify admissions or visits at higher complexity levels. However, the lack of standardized methodologies to measure upcoding hinders researchers and policymakers from fully understanding its prevalence, the medical conditions especially likely to be upcoded, the types of hospitals and geographic areas with high upcoding rates, and the overall impact of upcoding on health care spending. To address this gap, the authors of this report provide a review of methodologies to measure upcoding using literature from 2000 to 2023. Using seven electronic databases, the authors identified 39 articles that measure upcoding, which they classified into the following categories: variations in payments (n = 14 studies), variations in provider and patient attributes (n = 6), prediction algorithms (n = 6), self-reports (n = 3), and validations (n = 10). In most of these studies (n = 26), researchers examine upcoding at aggregated levels (e.g., frequency of upcoding at hospitals or in geographic areas) rather than identifying individually upcoded admissions (n = 13). Only three studies measure upcoding by comparing coding intensity with a gold standard, while the rest examine upcoding indirectly via measures of coding intensity. In their conclusion, the authors recommend that researchers and policymakers designing studies on providerbased upcoding consider outcome measures based on severity levels, payment changes that incentivize upcoding, provider and patient characteristics associated with upcoding, and indicators in administrative claims or electronic health record data.This research was funded by the National Institute for Health Care Reform and carried out within the Payment, Cost, and Coverage Program in RAND Health Care.RAND Health Care, a division of the RAND Corporation, promotes healthier societies by improving health care systems in the United States and other countries. We do this by providing health care decisionmakers, practitioners, and consumers with actionable, rigorous, objective evidence to support their most complex decisions. For more information, see www.rand.org/health-care, or contact