As a sole proprietorship, the Employer Identification Number (EIN) is generally used when you have employees. However, if you are an entity such as a LLC, corporation, partnership, etc., then it is mandatory that you obtain an EIN.
If you do not have employees, and you are filing as a self-employed individual, you will input your Social Security Number (SSN) in the return. Your SSN will carry over to the wage section of the return (Form W-2) and the 1040 Schedule C or Schedule C-EZ for your business income and expense reporting.
You will not begin using your EIN until you have employees (the number is then reported on the payroll tax returns). If the nature or structure of your business changes to one of the aforementioned entities, then you will be required to obtain a new EIN. The EIN is then reported on all your applicable tax filing forms.
