The athletics world has been shocked by new evidence that suggests hundreds of competitors may have cheated to win medals.
Athletes push themselves to the limits to win competitions and are regularly tested to make sure they are not using anything illegal to improve their performance.
However a secret list has just been revealed which suggests that hundreds of athletes might have taken drugs, or banned supplements, to try to help them win.
It was shown to the Sunday Times newspaper in the UK, and a TV station in Germany, and reveals the results of huge numbers of abnormal blood tests going back to 2001.
The people who run the sport - the International Association of Athletics Federations - say they've always worked hard to make sure athletes can't cheat and will continue to carry out tough testing programmes.
Watch Martin's report to find out more...