Milosevic denied the claims on Thursday and accused the Western world of telling "an ocean of lies" about him.
Huge graves were found
Some of the most horrific acts were said to have taken place in Kosovo, which was part of Serbia, the most powerful part of Yugoslavia in the 90s.
But Milosevic said many of the horror stories were made up to give countries like America an excuse to get involved in the Kosovan war, because they didn't agree with Milosevic's ideas.
And even though huge graves were found in Kosovo full of people killed in the fighting, Milosevic claimed the bodies were "placed there" to make him look guilty.
It's one of the most important political trials in recent years and is expected to last until 2004.