Large Database and Registry Research in JointArthroplasty and Orthopaedics T he digital revolution has made possible the proliferation of large databases and the mining of "big data" from many sources in orthopaedic surgery. This revolution in the compiling of, and access to, huge amounts of data has in turn led to dramatic advances in the types of studies and research that can be done in our field. A symposium/workshop was held on October 14 and 15, 2021, in Chicago, Illinois, to explore the power and potential of large databases; consider the weaknesses and risks of analyses using these databases; provide information on methods, specific attributes, and best uses of the most commonly used databases; and provide guidance on how best to select and use these databases for orthopaedic research. The JBJS Supplement on Large Database and Registry Research in Joint Arthroplasty and Orthopaedics is the product of that symposium.The idea behind the symposium was that a systematic evaluation of the state of large-database research in orthopaedic surgery could provide much valuable information to the many stakeholders who interact with these databases. These stakeholders include orthopaedic researchers, orthopaedic surgeons who read orthopaedic research, and journal reviewers and editors who evaluate research. The symposium concentrated on databases commonly used in joint arthroplasty, but much of the information presented in the JBJS Supplement has applicability across other orthopaedic disciplines.