100 m (one hundred meters) is the shortest outdoor sprint race distance in athletics. The reigning 100 meter Olympic champion is often named "the fastest man/woman in the world", even though the world record for the 200 meter race has had a faster average speed in the men's race since the mid 1990s.

In the past, athletes often competed over 100 yards (91 m) instead of the 100 m, especially in the United States. This shorter distance is now obsolete. Indoors events are normally held over 60 m (sometimes 50 m or 55 m) as few facilities have a 100 m indoor straight.

On an outdoor 400 meter running track, the 100 m is run on the home straight, the start being set on an extension to make it a straight-line race.

Major 100m races, such as at the Olympic Games, attract much attention, particularly when the world record is thought to be within reach.

The men's world record has been improved upon twelve times since the introduction of electronic timing in 1968, never being surpassed by more than 0.05 s at a time. The current men's world record of 9.69 seconds, is held by Usain Bolt of Jamaica, set at the 2008 Beijing Olympic Games final on August 16, 2008. The current women's world record of 10.49 seconds, was set by Florence Griffith-Joyner, in Indianapolis, Indiana, on 16 July 1988.

Illegal drug use has been seen by some people as a means to gain a competitive edge; in particular, the scandal at the 1988 Summer Olympics when the winner Ben Johnson was stripped of his medal. In 2003 revelations of failed drug tests by sprinting legend Carl Lewis before the 1988 Seoul Olympics also put the validity of his achievements into question.

At the start, some athletes play psychological games such as trying to be last to the starting blocks, although direct intimidation would be considered unsportsmanlike. The starter will keep the sprinters in the set position for an unpredictable time of around two seconds and then fire the starting gun.

The time between the gun and first kick against the starting block is measured electronically, via sensors built in the gun and the blocks. A reaction time less than 0.1 seconds is considered a false start. The 0.1-second interval accounts for the sum of the time it takes for the sound of the starter's pistol to reach the runners' ears, and the time it takes to react to it.

For many years a sprinter was disqualified if responsible for two false starts individually. However, this rule allowed some major races to be restarted so many times that the sprinters started to lose focus. The current rule, introduced in February 2003, is that, after one false start, anyone responsible for a subsequent false start is disqualified immediately. This rule has led to some sprinters deliberately false-starting to gain a psychological advantage: an individual with a slower reaction time might false-start, forcing the faster starters to wait and be sure of hearing the gun for the subsequent start, thereby losing some of their advantage. In order to avoid such abuse, the IAAF considered in 2005 to change the rule so that the first false starting athlete is immediately disqualified, but many countries objected as it would not leave any room for innocent mistakes. Justin Gatlin commented, "Just a flinch or a leg cramp could cost you a year's worth of work."

Climatic conditions are a crucial factor for good performances in the 100 m. Air resistance is the primary climatic factor in sprint performances. A strong head wind is very detrimental to performance, while a tail wind can improve performances significantly. For this reason, a maximum tail wind of 2.0 m/s is allowed for a 100 m performance to be considered eligible for records, or "wind legal". The fastest recorded time for the distance, although excluded from the records because of wind assistance, was 9.68 s by Tyson Gay of the United States on 29 June 2008 during the 2008 U.S. Olympic Track & Field Trials at Hayward Field in Eugene, Oregon; the tail wind speed was 4.1 m/s, more than double the IAAF legal limit.

Furthermore, sprint athletes perform better at high altitudes because of the thinner air, which provides less air resistance. While there are no limitations on altitude, performances made at altitudes greater than 1000 m above sea level are marked with an "A".