It's a condition that affects the way you live your life and see the world around you.
Our presenter Rosie was born with autism. She says even though living with it is difficult, it makes her unique and who she is.
As well as telling her own story, you'll find out how Ben, Tony and Lenny are affected by the condition.
Lots of children with autism get bullied at school and can find it hard to get jobs when they grow up.
In our special film Rosie wants to tell the world what autism is and what it's like to live with it.
Newsround has made a special programme about autism.