It works to reduce the size of a hat.  The instructions say to place it on the inside of the sweatband.  This is a little ambiguous, and it actually works best if the adhesive side is place on the inside of the hat, rather than the inside of the sweatband.  The adhesive is not very resistant to sweat and oil.  Removing the paper backing is an exercise in frustration.  I had to use a pocketknife and tweezers to separate the backing from the adhesive.  It shouldn't be that hard to make it easier to remove the backing.