A Federal Employer Identification Number (FEIN)- sometimes referred to as an EIN or Tax ID number is essential if you want to run your business in America. This nine-digit number is assigned to you by the Internal Revenue Service (IRS) for tax filing and reporting purposes. With it, you can also open a US bank account, hire employees, contribute toward employee 401(k) plans, and even file for bankruptcy. While most businesses must have an EIN, this is optional for some single-owner companies.