Thousands of top athletes from all over the world will be going for gold at the Olympic Games in Beijing this summer.
Competition in the 28 different sports is so tough that some competitors take drugs to make themselves faster or stronger, in their bid to win.
Newsround's Olympics reporter Sonali has been finding out what's being done to stop the cheats and try to keep the Games drug-free.
Canadian sprinter Ben Johnson was the first big star to be caught using drugs.
American sprinter Marion Jones
He used them at the Seoul Olympics in 1988, but since then loads more athletes have cheated their way to world records or Olympic medals.
American sprinter Marion Jones won five medals at the Sydney Games in 2000, but had to give them all back after admitting taking drugs.
And there were plenty more in Athens, in 2004, including a Greek weightlifter called Leonidas Sampanis.
Athletes are banned from taking thousands of drugs that could make them bigger, stronger and faster because it would give them an unfair advantage over anyone who hasn't taken them.
Even some everyday drugs like asthma inhalers and some cough and cold medicines aren't allowed.
If athletes need to take any of these, they have to get special permission from an organisation called UK Sport, which is in charge of making sure British athletes don't take drugs.
But how easy is it to catch the cheats?
I asked Joe Marshall from UK Sport who said they could test any athlete, anytime, anywhere.
"We can knock on their door at 6am and say we want to do a test," he said. "We feel confident that if an athlete is taking drugs, we can find them."
Testing for drugs in sport was introduced more than 50 years ago.
The athletes' wee is tested for signs of drugs
I visited the Drug Control Centre at King's College in London, which is testing every British athlete going to Beijing for the Olympic and Paralympic Games.
Each athlete has to provide them with a sample of their wee and Professor David Cowan, from the lab, told me what they do with it.
"We take some of the wee and go through a number of different tests," he said. "We put the samples into lots of different tubes and we add different chemicals to it.
"Finally a machine tells us what's in the wee sample. That's how we can tell if someone is taking drugs and we've found a cheat."
If anyone is found to be cheating, they're reported to the people in charge of sport in the UK.
And as far as they're concerned, there's no place for athletes who take drugs to hide.