The famous highway originally ran from Chicago, Illinois, through Missouri, Kansas, Oklahoma, Texas, New Mexico, Arizona, and California, before ending at Los Angeles, encompassing a total of 2,448 miles. It was recognized in popular culture by both a hit song and a television show in the 1950s and 1960s. More recently, U.S. Route 66 was referenced in the 2006 Pixar animated film Cars.

Route 66 underwent many improvements and realignments over its lifetime, changing its path and overall length. Many of the realignments gave travelers faster or safer routes, or detoured around city congestion. One realignment moved the western endpoint further west from downtown Los Angeles to Santa Monica.

Route 66 was a major path of the migrants who went west, especially during the Dust Bowl of the 1930s, and 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 even with the growing threat of being bypassed by the new Interstate Highway System.

US 66 was officially removed from the United States Highway System on June 27, 1985, after it was decided the route was no longer relevant and had been replaced by the Interstate Highway System.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". It has begun to return to maps in this form. Some portions of the road in southern California have been redesignated "State Route 66", and others bear "Historic Route 66" signs and relevant historic information.