400 metres

The 400 metres, or 400-meter dash, is a sprint event in track and field competitions. It has been featured in the athletics programme at the Summer Olympics since 1896 for men and since 1964 for women. On a standard outdoor running track, it is one lap around the track. Runners start in staggered positions and race in separate lanes for the entire course. In many countries, athletes previously competed in the 440-yard dash (402.336 m)—which is a quarter of a mile and was referred to as the "quarter-mile"—instead of the 400 m (437.445 yards), though this distance is now obsolete.

Like other sprint disciplines, the 400 m involves the use of starting blocks. The runners take up position in the blocks on the "ready" command, adopt a more efficient starting posture which isometrically preloads their muscles on the "set" command, and stride forwards from the blocks upon hearing the starter's pistol. The blocks allow the runners to begin more powerfully and thereby contribute to their overall sprint speed capability. Maximum sprint speed capability is a significant contributing factor to success in the event, but athletes also require substantial speed endurance and the ability to cope well with high amounts of lactic acid to sustain a fast speed over a whole lap. While considered to be predominantly an anaerobic event, there is some aerobic involvement and the degree of aerobic training required for 400-metre athletes is open to debate.

The current men's world record is held by Wayde van Niekerk of South Africa, with a time of 43.03 seconds. Van Niekerk is also the Olympic record holder. Steven Gardiner is the reigning Olympic Champion. Antonio Watson is the current world champion. Christopher Morales Williams is the world indoor record holder with a time of 44.49 seconds. The current women's world record is held by Marita Koch, with a time of 47.60 seconds. Shaunae Miller-Uibo is the reigning women's Olympic champion. Marileidy Paulino is the current world champion. Femke Bol holds the women's world indoor record at 49.17 (2024). The men's T43 Paralympic world record of 45.07 seconds is held by Oscar Pistorius.

An Olympic double of 200 metres and 400 m was first achieved by Valerie Brisco-Hooks in 1984, and later by Marie-José Pérec of France and Michael Johnson from the United States on the same evening in 1996. Alberto Juantorena of Cuba at the 1976 Summer Olympics became the first and so far the only athlete to win both the 400 m and 800 m Olympic titles. Pérec became the first to defend the Olympic title in 1996, Johnson became the first and only man to do so in 2000. From 31 appearances in the Olympic Games, the men's gold medalist came from the US 19 times.