Frontier Airlines, Inc. is a current operating brand of Republic Airways Holdings. Headquartered in Indianapolis, Indiana, United States, the carrier operates flights throughout the United States, Mexico, and Costa Rica. Two-thirds of Frontiers domestic destinations are west of the Mississippi River. Frontiers Interactive Route Map shows the carrier maintains hubs (airline hub) at Denver International Airport, Milwaukee General Mitchell International Airport, and focus city Kansas City International Airport. It provides regional service to the surrounding Rocky Mountain States (Mountain States) through a code-share agreement with Great Lakes Airlines. Frontier is not a member of an air carrier alliance (airline alliance).

In March 2007, Republic Airlines slowly began replacing Horizon Air as the operator of their feeder service, Frontier Airlines operated by Republic Airlines; Frontier JetExpress, which was operated by Horizon Air, ceased operations on November 30, 2007. In April 2008, Frontier and Republic terminated their agreement and Republic withdrew its aircraft from Frontier service on June 23, 2008.

On April 10, 2008, Frontier announced that it filed for Chapter 11 bankruptcy due to its credit card processor, First Data, attempting to withhold significant proceeds from ticket sales. First Data decided that it would withhold 100% of the carriers proceeds from ticket sales beginning May 1. According to Frontiers press release, "This change in practice would have represented a material change to our cash forecasts and business plan. Unchecked, it would have put severe restraints on Frontiers liquidity..." Its operation continues uninterrupted, though, as Chapter 11 bankruptcy protects the corporations assets and allows restructuring to ensure long-term viability. In January 2010 Republic Airways announced that it will move all of its executives to Indianapolis. In February the Denver Business Journal stated that the headquarters would be moved "soon."