The Hillsborough disaster was one of the worst sporting tragedies in British history.
On 15 April 1989, 96 Liverpool football fans were killed by a crush at the Sheffield Wednesday stadium, during an FA Cup semi-final.
There was a big investigation at the time - but not all the details were made public by the government.
Now, MPs will debate whether to release thousands of documents about the investigation.
At the time, no-one really knew what had caused the disaster.
Immediately afterwards, some people blamed the fans, but later on a report criticised the design of the football ground itself and the way the police handled it.
The families of the victims now hope that this new debate will bring them some of the answers they have been looking for.