It really depends on how serious you will be about it... if it's just sort of a hobby and you're making a few hundred bucks then there isn't much to worry about... but if you're out there raking in bank and marketing your services "as a business" then that is a different story... most states/cities would require you to register your "business" as a sole proprietorship. You will also need to make sure you keep good records of all the freelance income and make sure you're paying the right amount of taxes. Depending on how much money you expect to make freelancing, you'll probably want to talk to a good CPA to make sure you cover all your bases.