Each swimming event is broken up into rounds with the purpose of trimming the field of competitors (which can sometimes number upwards of 90+ for individual events) down to a final race of eight. Individual events at distances of 200m and shorter require three rounds: preliminary heats, semifinals, and a final. 

What is a heat in swimming?

Technically, a heat can refer to any single race within a swimming event, regardless of round. However, for the most part, the term "heat" is used in reference to the preliminary (first) round of competition. All swimmers entered into a given event must swim in a preliminary heat to qualify for the next round of competition, whether it be the semifinal stage (events of 200m distance or less) or directly to the final (events longer than 200m, including all relays). The number of heats is determined by the number of athletes entered into the event. If 48 athletes are entered, six preliminary heats will take place since each heat can include up to eight swimmers.

Winning a heat does not automatically qualify a swimmer to the next stage. Only the swimmers with the fastest recorded times once all heats have taken place (top 16 if there is a semifinal stage, top eight if the event goes directly to the final) will advance. At the Olympics, the fastest swimmers are generally held until the last handful of heats. As a result, winners of earlier heats frequently do not advance.

Olympic swimming competition days generally contain two sessions: one for only preliminary heats, and a second for semifinals and finals. The exceptions are the first day of competition, which lacks a semifinal/final session, and the last day, which does not contain a heats session. 

How do swimmers qualify for the final?

For events longer than 200m in distance, the eight fastest swimmers from the preliminary heats automatically qualify for the final, where medals are at stake. 

For events 200m and under, the 16 fastest swimmers from the preliminary heats are sorted into two semifinals. After both semifinals have taken place, the swimmers with the eight fastest times advance to the final. As with the preliminary heats, winning a semifinal does not guarantee advancement to the final.

What if there is a tie?

In the event that two or more swimmers vying for the final spot in the next round record identical times, those athletes swim an additional head-to-head race to determine who advances. This is also known as a "swim off."

If there is a tie in the final for one of the medal places, the tied athletes are all awarded medals of the same color. If two athletes tie for silver, for example, both receive silver medals and no bronze medal is awarded for that event.

Swimmers and relay teams are seeded according to times from the previous round so that the fastest swimmers/teams are in the central lanes of the pool, and the slowest are in the outer ones. The lanes are assigned in the following order:

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Lane 1: Seventh-fastest time                                                           
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Lane 2: Fifth-fastest time
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Lane 3: Third-fastest time
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Lane 4: Fastest time 
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Lane 5: Second-fastest time
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Lane 6: Fourth-fastest time
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Lane 7: Sixth-fastest time
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Lane 8: Slowest time
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Being in the middle of the pool is advantageous because waves reverberating off the side walls can slow a swimmer. Also, swimmers in the middle lanes are better able to monitor other competitors. 

Each country can enter up to eight swimmers in each relay. In many cases, a country will have alternates swim in the prelims, allowing the fastest swimmers to be rested for the final. Athletes who swim in only the heats are awarded a medal if their country earns one in the final, meaning up to eight swimmers can win a medal for a relay.  

The 10k open water race only has a final, which will be swam in the Seine river at the Paris Games. The event was introduced at the 2008 Beijing Olympics and is sometimes referred to as the "marathon swim" or "the 10k." Twenty-two competitors will swim in the event. There are no lanes, and physical contact between swimmers is expected, especially during the mass-start beginning to the race. Like the running marathon, the open water race follows a prescribed course. While the course usually includes multiple laps, the Paris Games will feature the first point-to-point 10k open water race in the history of the Olympics. There are two 10k races: a men’s race and a women’s race.