Most business filings are done at the state or county level depending upon the nature of your business entity (corporation, LLC, partnership, sole proprietorship, etc.). Usually, although not always, a single filing is sufficient for you to conduct business anywhere in the state in which you file. If you are conducting business in more than one state, the answer will depend on the level of contact you have with the other state(s). If your business has a physical presence in the other state(s), most states will require a filing similar to the one in your home state. If you merely have customers in a particular state with whom you do business by phone, e-mail, regular mail, or occasional face to face meetings, you most likely would not have to file (but be aware that you might be subject to being sued in that state by a displeased customer or supplier). An attorney who practices in this area should be able to help you comply with all applicable filing requirements.