It’s well-known, counterfeit money exists and is in circulation in the U.S.  The United States Government estimates less than .0001% of all money in circulation is counterfeit. 

Take a look at a humorous MSNBC story about a lady who tried to use a one million dollar bill at Wal-Mart.

All bills except for the $1 and $2 dollar bill have significant security features that have helped eliminate counterfeit money. New security features are added every few years to keep counterfeiters behind.  The U.S. Secret Service handles counterfeiting as a Federal Felony which is punishable by a fine and jail time.

To learn more about Counterfeit money in the U.S., visit Howstuffworks.