Legally, you are required to identify your business with one of two numbers: either your Social Security Number or an EIN (Employer Identification Number, a.k.a. Federal Tax ID Number). If you are a sole proprietor, your Social Security Number can be used on all of your government forms and other official documents, but most small business advisors recommend that you apply for an EIN and use that number instead. If you are a corporation, LLC, or other state-level entity, you must obtain an EIN because your business is an entirely separate legal entity. For more information on EINs, please click here.