An EIN (Employer Identification Number) is a unique number given by the government to use by businesses for the purpose of identification. As a sole proprietor, you can elect to either use your SS# or apply for an EIN number to use on business documents. If you plan to operate more than one business as a sole proprietor, you do not have to apply for a second EIN number. Your single EIN will be used for any future sole-proprietorships you may wish to create. I’ve elected to create a post on this because the IRS does not list this on their website (at least not that I could find) and the only way to find out is to call the IRS help desk, but seriously, who wants to do that?