Football at the Olympics is a lot like the World Cup, but with fewer teams.
In the men's event 16 teams will play to succeed Argentina as the 2004 Olympic champions.
There is a special rule for the Olympics, as most players in each team must be younger than 23. However, each country can select three 'overage' players who are older than that.
In the women's event there are 10 teams and the only restriction on players is that they must be older than 16.
Women's football has only been an Olympic sport for the last three Games, with Norway winning in Sydney and the US in 1996.
Football is a massive sport in the US for girls, and their team always does well at major championships and won in 2004 at Athens.