If you use the My Fitness Pal app - there's some bad news - you might have been hacked!
Personal information has been stolen from around 150 million users worldwide.
Under Armour, which owns the weight-loss service, says the data breach involves account names, email addresses and scrambled passwords.
It's the largest hack this year and one of the top five to date.
Anyone with an account is being told to change their passwords.
Here's the information Under Armour sent out to the app's users: