U.S. Route 66 (US 66 or Route 66), also known as the Will Rogers Highway and colloquially known as the Main Street of America or the Mother Road, was one of the original highways within the U.S. Highway System. Route 66 was established on November 11, 1926—with road signs erected the following year. The highway, which became one of the most famous roads in America, originally ran from Chicago, Illinois, through Missouri, Kansas, Oklahoma, Texas, New Mexico, and Arizona before ending at Los Angeles, California, covering a total of 2,448 miles (3,940 km). It was recognized in popular culture by both a hit song and the Route 66 television show in the 1960s.

Route 66 served as a major path for those who migrated west, especially during the Dust Bowl of the 1930s, and it supported the economies of the communities through which the road passed. People doing business along the route became prosperous due to the growing popularity of the highway, and those same people later fought to keep the highway alive in the face of the growing threat of being bypassed by the new Interstate Highway System.

Route 66 underwent many improvements and realignments over its lifetime, and it was officially removed from the United States Highway System on June 27, 1985 after it had been replaced in its entirety by the Interstate Highway System. Portions of the road that passed through Illinois, Missouri, New Mexico, and Arizona have been designated a National Scenic Byway of the name "Historic Route 66", which is returning to some maps. Several states have adopted significant bypassed sections of the former US 66 into the state road network as State Route 66.