Regional airlines are airlines that operate regional aircraft to provide passenger air service to communities without sufficient demand to attract mainline service. There are two main ways for a regional airline to do business:

As an affiliated airline, contracting with a major airline, operating under their brand name (for example, Endeavor Air operates flights under the Delta Connection brand name for Delta Air Lines), and filling two roles: delivering passengers to the major airline's hubs from surrounding towns, and increasing frequency of service on mainline routes during times when demand does not warrant use of large aircraft, known as commuter flights.
Operating as an independent airline under their own brand, mostly providing service to small and isolated towns, for whom the airline is the only reasonable link to a larger town. Examples of this are PenAir, which links the remote Aleutian Islands to Anchorage, Alaska, and Mokulele Airlines, which operates in the Hawaiian islands.Small regional airlines operating in the U.S. during the 1960s and 1970s were also known as commuter airlines and were classified as such in the Official Airline Guide (OAG).