The national championship was sanctioned by the
Contest Board of the
American Automobile Association (AAA). The AAA first sanctioned automobile motorsports events in 1902. At first it used the rules of the Automobile Club of America (ACA), but it formed its own rules in 1903. It introduced the first track season championship for racing cars in
Barney Oldfield was the first champion. No official season championship was recognized from 1906-1915, however, single races were held. Official records regard
1916 as the next contested championship season.
 Years later, retroactive titles were named back to 1902.
 These post-factum seasons (1902-1904, 1906-1915, and 1917-1919) are considered unofficial and
revisionist history by accredited historians.
Racing did not cease in the United States during
WWI, but the official national championship was suspended. The
Indianapolis 500 itself was voluntarily suspended for 1917–1918 due to the war. In 1920, the championship officially resumed, and despite the difficult economic climate that would later follow, ran continuously throughout the
Depression. Shortly after
Pearl Harbor, all auto racing was suspended during World War II. From 1942 to 1945 no events were contested, banned by the U.S. government primarily on account of
rationing. Racing resumed in full in 1946. The
1946 season is unique, in that it included six Champ Car events, and 71 "
Big Car" races, as organizers were initially unsure about the availability of cars and participation.
AAA ceased participation in auto racing at the end of the 1955 season. It cited a series of high-profile fatal accidents, namely
Bill Vukovich at
Indianapolis, and the
Le Mans disaster.
Through 1922 and again from 1930 to 1937, it was commonplace for the cars to be two-seaters, as opposed to the aforementioned standard single-seat form. The driver would be accompanied by a
riding mechanic (or "mechanician").
The national championship was taken over by the
United States Auto Club (USAC), a new sanctioning body formed by the then-owner of the
Indianapolis Motor Speedway,
Tony Hulman. Championship racing continued to grow in popularity in a stabilized environment for over two decades, with the two traditional disciplines of paved oval tracks and dirt oval tracks. During the 1950s, front-engined "roadsters" became the dominant cars on the paved oval tracks, while "upright" Champ Dirt Cars continued to dominate on dirt tracks. In the 1960s, drivers and team owners with road racing backgrounds, both American and foreign, began creeping into the series and the paved oval track cars evolved from front-engine "
roadsters" to rear-engine
formula-style racers. Technology, speed, and expense climbed at a rapid rate. The schedule continued to be dominated by oval tracks, but a few road course races were added to assuage the newcomers. Dirt tracks were dropped from the national championship after 1970.
During the 1970s, the increasing costs began to drive some of the traditional USAC car owners out of the sport. The dominant teams became Penske, Patrick, Gurney, and McLaren, all run by people with road racing backgrounds. There was a growing dissent between these teams and USAC management. Events outside Indianapolis were suffering from low attendance, and poor promotion. The Indy 500 was televised on a same day tape delayed basis on ABC, however, most of the other races had little or no coverage on television.
Towards the end of the decade, the growing dissent prompted several car owners to consider creating a new sanctioning body to conduct the races. Meanwhile, two events had a concomitant effect on the situation.
Tony Hulman, president of the Indianapolis Motor Speedway and founder of USAC, died in the fall of 1977. A few months later, eight key USAC officials were
killed in a plane crash. By the end of 1978, the owners had broken away and founded Championship Auto Racing Teams (CART) to wrest control of Championship racing away from USAC.
CART & USAC (1979–1981): First open wheel "split"
Championship Auto Racing Teams (CART) was formed by most of the existing team owners, with some initial assistance from the
SCCA (in order to be recognized by
ACCUS). Therefore, there were two national championships run each by USAC and CART. The Indianapolis 500 remained under USAC sanction. The top teams allied to CART, and the CART championship quickly became the more prestigious national championship. USAC ran a "rump" 1979 season, with few name drivers — the only exception being
A. J. Foyt. In 1979, USAC denied several of the entries from the CART teams at the
1979 Indianapolis 500. The controversy saw a court injunction during the month, which allowed the CART-affiliated entrants to participate.
In 1980 USAC and CART jointly formed the Championship Racing League (CRL) to jointly run the national championship, but
IMS management disliked the idea. USAC pulled out of the CRL arrangement in July.
 CART continued with the schedule for the remainder of the season.
 Both CART and USAC awarded separate national championship titles that year, and
Johnny Rutherford happened to win both.
In 1981–1982, the Indianapolis 500 remained sanctioned by USAC. The preeminent national championship was now the one being sanctioned by CART. The Indy 500 field would consist largely of CART teams, as well as numerous independent, "Indy-only" teams. Indianapolis was not included as a points-paying round of the CART national championship. In addition, by that time USAC had designated Indianapolis an "invitational" race, offering entries only to invited teams.
 That moved in part to prevent the uproar over denied entries which occurred in 1979. One further race in 1981 was run by USAC at
Pocono. This race was not supported by many CART teams, and featured a mixed field filled out by converted dirt track cars. USAC soon stopped sanctioning championship races outside the Indianapolis 500.
CART & USAC (1982–1995)
Stability returned and the national championship was now run by CART full-time. The Indianapolis 500 was sanctioned singly by USAC, but points were paid towards the CART season championship. The cars and engines used in the CART races and USAC-sanctioned Indy 500 were the same, with only relatively minor rules differences. The Indy 500 field would consist of the CART regulars, and numerous one-off ("Indy only") entries. On occasion, some of the "Indy only" entries also elected to participate in the
Michigan 500 and
Pocono 500 (both sanctioned by CART) given the increased stature and exposure of those two events.
USAC's Gold Crown Championship continued, settling into an unusual June through May schedule calendar. This provided that the Indianapolis 500 would be the final race of the respective season. However, during that period, the USAC Gold Crown schedule never included more than one race (i.e., Indianapolis). As such, the winner of the Indy 500 would automatically win the USAC Gold Crown Championship.
CART & IRL (1996–2003): Second open wheel "split"
Tony Hulman's grandson,
Tony George, president of the Indianapolis Motor Speedway, founded the
Indy Racing League (IRL), to begin competition in 1996. It would exist as a separate championship, and leveraged the fame of the Indianapolis 500, which was placed as its centerpiece. The CART teams interpreted the move as a perceived "lockout", and boycotted the 1996 Indy 500 as a result. It was the beginning of the second open wheel "split." Initially, USAC sanctioned the IRL, however after officiating controversies in 1997 at Indianapolis and
Texas, the USAC was replaced by the IRL's in-house officiating.
CART, which had been licensing the trademarked "IndyCar" name for several seasons, subsequently entered into a legal battle with the Indianapolis Motor Speedway (the trademark owner) over the use of the moniker. Eventually a settlement was reached in which CART gave up use of the name, but the IRL in turn could not use it until 2003. CART rebranded themselves with the CART name, and began referring to their machines as Champ Cars.
CART's existing national championship remained dominant after the split for some time, initially retaining the top drivers, teams, and sponsors. In 1998, CART went public and raised $100 million USD in its
stock offering. However, in 2000, CART teams began to return to the Indy 500, eventually defecting permanently to the IRL. CART also suffered negative publicity over the cancellation of the
Firestone Firehawk 600 in 2001. For 2003, it lost title sponsor
FedEx and engine providers
Toyota to the IRL.
IRL IndyCar Series & Champ Car World Series (2004–2007)
After steadily losing teams and drivers, sponsors, and manufacturers, and after a series of major financial setbacks, CART filed for bankruptcy in 2003. The assets were purchased by a consortium called Open Wheel Racing Series (OWRS) in 2004 and the series was renamed the Champ Car Open Wheel Racing Series, later renaming it to
Champ Car World Series. However, the sanctioning body continued to be plagued by financial difficulties, In 2007, CCWS's presenting sponsors
Ford Motor Company withdrew and CCWS lacked the resources to mount the 2008 season.
During this time, the IRL was now operating under the moniker IndyCar Series, and slowly beginning to establish itself as the more preeminent national championship trail. In 2005, the IRL added road/street courses, and began picking up several former CART venues.
IndyCar (2008–present): Unification era
Prior to the start of the 2008 season, the CCWS Board authorized bankruptcy and Champ Car was absorbed into the IRL, creating a unified series for the national championship for the first time since 1978. The unified series competed under the name
IndyCar Series. The two calendars were merged into one schedule, with the top Champ Car races surviving. Some of the other races from the Champ Car schedule were dropped or put on hiatus for a few seasons.
All historical record and property of CART/CCWS was assumed by the IRL. In 2011, the sanctioning body dropped the Indy Racing League name, becoming IndyCar to reflect the merged series. The series operated under the name
IZOD IndyCar Series from 2010-2013, then became known as the
Verizon IndyCar Series starting in 2014.