When you purchase a bike in Hawaii, you are required to pay a one-time tax (aka bicycle registration fee). It isn't much - I think it's about $15 or so. And they issue the owner a big, ugly sticker that is supposed to be stuck on the frame. If you don't have that sticker on, the police have a right to repossess your bike until you pay the tax or attach the sticker. It's one of those laws that's rarely enforced. I've never attached that ugly sticker to any of my bikes and haven't run into a problem yet.