Have you ever thought that an app was free, but it still ended up costing you or your parents a lot of money?
Newsround asked tech blogger Tom Cheesewright to give you some top tips to avoid unexpected charges:
1. Even free apps often have extras that cost.
Tell your parents about it - it's quite possible that you know more than they do.
2. You might know a password, but think twice about using it.
If you have to put in a password there's a good chance something is going to cost money.
3. Suggest to your parents that they might put some rules in place to prevent accidental spending - it's easy to do.
4. Games on tablets and smartphones can be a good thing to do together, not just on your own.
Until you're really confident with what you're doing, ask an adult to play along with you - they'll probably learn a lot, and enjoy it too!