"Upgauging” is an airline industry technique enabling air carriers to increase capacity by adding seats to existing jets and replacing smaller planes with larger ones. While these practices are generally the result of airline network and system-wide strategies, their impacts are often experienced at the local level by the airport community. Airport Cooperative Research Program (ACRP) Synthesis 97: How Airports Plan for Changing Aircraft Capacity: The Effects of Upgauging explores a broad concept of airline upgauging taking into account the principal drivers and techniques of upgauging, from both airline and airport perspectives. This study is based on information acquired through a literature review, survey results from 18 airports participating in the study that experienced major variations in passenger enplanements over the previous 5 to 10 years, and interviews with representatives of five airports and four state transportation agencies.