For US tax purposes, an EIN or Employment Identification Number is a unique 9-digit identifying the number for business to identify themselves in the eyes of the government. No matter how small or big your company is; when paying tax to the Internal Revenue Services (IRS) you should have an EIN. However, businesses owned by a single person can choose to pay their tax under their personal Social Security Number. A business that needs an EIN and does not feel comfortable using their Social Security Number can apply for EIN online to start their company and be sure they are safe the IRS.