An Employer Identification Number (EIN) is a unique number assigned by the IRS to identify a business for tax purposes. Think of it like a Social Security number for a company. If you’re setting up a Solo 401k or any retirement plan for your business, you’ll usually need an EIN—even if you’re the only employee. It’s used when opening a retirement account, filing certain tax forms, or handling payroll. Getting an EIN is free and can be done quickly through the IRS website.