The X Factor star Cher Lloyd has been talking about when she was bullied online.
She said she didn't talk to anyone about it because she thought she could handle it on her own.
But she says: "I think that's the worst thing you could possibly do."
Cyberbullying is when children or teenagers use the web, or mobile phones or any other technology to threaten, tease or embarrass another young person.
Cher has been talking about this because a children's charity called Beatbullying says it's a big problem.
For more information look at the links on the left, including where to get help if you are being bullied.
Cher Lloyd's interview is on Panorama on BBC One 8.30pm on Monday.