High Quality Content by WIKIPEDIA articles! Frontier Airlines, Inc. is a low-cost airline and the current operating brand of Republic Airways Holdings. Headquartered in Denver, Colorado, United States, the carrier operates flights throughout the United States, Mexico, and Costa Rica. Two-thirds of Frontier's domestic destinations are west of the Mississippi River. The carrier maintains its sole hub at Denver International Airport, and provides regional service to the surrounding Rocky Mountain States through a code-share agreement with Great Lakes Airlines. Frontier is not a member of an air carrier alliance.