Rio 2016 officially gets under way with the opening ceremony on Friday - but the action kicks off on two days before that with the football.
The first group games in the women's tournament take place on Wednesday - so-called day minus 2 - with Sweden facing South Africa at 17:00 BST.
The men start on Thursday, when Iraq play Denmark (17:00 BST).
Here's all you need to know...
Why does football start early at the Olympics?
It's simply for scheduling reasons, as it was at London 2012.
With 16 men's and 12 women's teams, there are 58 matches to get through, with only six rest days.
Group A: Brazil, South Africa, Iraq, Denmark
Group B: Sweden, Colombia, Nigeria, Japan
Group C: Fiji, South Korea, Mexico, Germany
Group D: Honduras, Algeria, Portugal, Argentina
Group E: Brazil, China, Sweden, South Africa
Group F: Canada, Australia, Zimbabwe, Germany
Group G: USA, New Zealand, France, Colombia
Which stars are in action?
Barcelona forward Neymar will captain a talented Brazil squad aiming to win its first Olympic gold medal.
He is joined by Lazio midfielder Felipe Anderson, Barca club-mate Rafinha and Paris Saint-Germain defender Marquinhos.
Tottenham forward Son Heung-min will represent South Korea and Chelsea's John Mikel Obi brings experience for Nigeria.
Marta is one of the stars of the women's game, having been named world player of the year five times.
She competes in her fourth Olympics for Brazil alongside 38-year-old team-mate Formiga, who is going to her sixth Games and looking to continue her streak of appearing at every women's Olympic football tournament.
Reigning Olympic champions the USA have seven gold medal winners in their line-up. Among them is goalkeeper Hope Solo, who could win a record fourth Olympic title at Rio, and current world player of the year Carli Lloyd.
Day-by-day guide to what's on
Olympic football schedule
Why aren't Great Britain there?
Team GB fielded men's and women's teams at London 2012, with both sides reaching the quarter-finals.
There could have been a GB women's team to Rio - thanks to England's third-place finish at the 2015 World Cup - but there was not enough support among the home nations to make it a reality.
England backed the idea but no-one else did. Fifa was also unwilling to accept a Great Britain team without players from every home nation.
"It's disheartening they couldn't get a side together," said former England striker Kelly Smith, who was part of Team GB's 18-strong squad at London.
"The Americans view this as the ultimate. It's a great experience and you get to play against the best teams and players from across the world. "
How big is Olympic football?
Even though it was not included in the Olympic programme until 1996, the women's competition is often seen as more prestigious than the men's, which is age-restricted. Only three men in each squad can be over the age of 23.
For many Brazilians, the men's football will be the highlight of the Games, given they have yet to win the gold medal and the tournament is on home soil.
Who are the favourites?
Brazil are the firm favourites to take the men's title after the disappointment of losing in the final at London 2012.
Defending champions Mexico could prove a difficult opponent again, while Germany also have a strong team.
In the women's event, the USA will be the team to beat after winning gold in 2012 and also coming out on top at the World Cup last year.
The European trio of France, Germany and Sweden will also be strong.
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