Trunk carriers or trunk airlines or trunklines or trunks, were the US scheduled airlines certificated in the period 1939–1941 by the Civil Aeronautics Authority (CAA) or its immediate successor, the Civil Aeronautics Board (CAB) after the passage of the 1938 Civil Aeronautics Act on the basis of grandfathering: those carriers that were able to show they performed scheduled service prior to the passage of the Act. During the regulated period (1938–1978) these carriers were an especially protected class, with the CAB regulating the industry in many respects in the interests of these companies, a form of regulatory capture. The importance of these carriers is reflected is shown that in 2024, the three largest airlines in the United States, American Airlines, Delta Air Lines and United Airlines were among the carriers certificated through this grandfathering in 1939.
The CAB tightly regulated the industry and categorized airlines by function, the name of the trunk carriers reflected their role, the airlines that flew the main domestic (or trunk) routes. By contrast, a later group of CAB-regulated domestic carriers, first certificated in a five-year period after World War II, were known as local service carriers or feeder carriers, again names reflecting their purpose within the CAB-regulated industry.