Newsround's been chatting to Katie Collett from The Diana Award, a charity that tackles bullying.
There are lots of different reasons why people might bully others, but whatever the reason, it's always wrong and you should tell someone you trust about it.
Bullying doesn't just happen face to face. It can happen anywhere - online, at school or at home.
If you see someone being bullied, you should tell your teacher, parent or guardian. There are lots of ways you can help as, Katie explains...