A National Olympic Committee (NOC) may enter up to 3 qualified athletes in each individual event if all athletes meet the entry standard during the qualifying period. An NOC may also enter a maximum of 1 qualified relay team per event. Under the universality rule, NOCs may enter athletes regardless of time (1 athlete per sex) if they have no athletes meeting the entry standard. This makes it possible for every nation to have a minimum of two representatives in the sport.
The qualifying system for 2020 saw fundamental changes from previous Olympic qualifying systems. Where 2016 qualification and previous editions were based on set qualifying times, 2020 qualification is primarily based on world ranking. The IAAF continues to set qualifying times, but these are "set for the sole purpose of qualifying athletes with exceptional performances unable to qualify through the IAAF World Rankings pathway." The number of entrants per event is capped, with different caps for different events varying from 24 athletes for the combined events to 80 athletes for the marathons.
The IAAF world rankings are based on the average of the best five results for the athlete over the qualifying period. The results are weighted by the importance of the meet.
The qualifying time standards and may be obtained in various meets during the given period that have the approval of the IAAF. The qualifying period for the 10,000 m, marathon, walks, and combined events is from 1 January 2019 to 29 June 2020 and for the remaining events, from 1 May 2019 to 29 June 2020. The most recent Area Championships may be counted in the ranking even if not during the qualifying period, if they would be among the top five results for that athlete.
For the relays, a maximum of sixteen qualified NOCs shall be entitled to each event. The top eight teams in each event at the 2019 World Championships in Athletics (held in Doha from 28 September–6 October 2019) guaranteed a spot on their respective NOCs for the Olympics. The remaining half in each event are selected according to IAAF World Ranking List as of 29 June 2020 based on the aggregate of the 2 fastest times achieved by NOCs during the given period.
NOCs with more than three qualified athletes in an individual event may select, using their own rules, athletes from among those qualified. For example, the United States selects athletes based on the result of the 2020 United States Olympic Trials event, but has a policy of entering every athlete so qualified. Sweden only enters athletes good enough to reach at least the eighth position, based on an assessment by the Swedish NOC.
Athletes must have been born before 1 January 2005 (that, be at least 16 years old at the end of 2020) to compete. Youth athletes (born in 2003 or 2004, age 16 or 17 at the end of 2020) cannot compete in the throwing events, combined events, marathons, race walks, or 10,000 metre events. Junior athletes (born in 2001 or 2002, age 18 or 19 at the end of 2020) cannot compete in the marathons or the 50 kilometre race walk.