Rio 2016 officially gets under way with the opening ceremony on Friday - but did you know football kicks off the action two days earlier?
The first group stages of the women's tournament take place on Wednesday - so-called day -2 - with Sweden facing South Africa, while the men's group stages start on Thursday, when Iraq play Denmark (both 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. There is football being played on 12 days of the tournament, with six rest days.
Men's groups
- 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
Women's groups
- Group E: Brazil, China, Sweden, South Africa
- Group F: Canada, Australia, Zimbabwe, Germany
- Group G: USA, New Zealand, France, Columbia
Which stars are in action?
Barcelona forward Neymar will captain a Brazil squad aiming to win its first Olympic gold medal.
The hosts also have Lazio midfielder Felipe Anderson in their squad along with Neymar's Barca team-mate Rafinha and Paris Saint-Germain defender Marquinhos.
Meanwhile, Tottenham forward Son Heung-min will represent South Korea and Chelsea's John Mikel Obi brings experience for Nigeria.
Five-time women's world player of the year Marta competes in her fourth Olympics for Brazil, while her 38-year-old team-mate Formiga is going to her sixth Games and is looking to continue her streak of appearing at every women's Olympic football tournament.
The reigning world and Olympic champions 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.