An employer identification number (EIN), also called a tax ID number or taxpayer ID, is required for most business entities. As its name implies, this is the number used by the Internal Revenue Service (IRS) to identify businesses with respect to their tax obligations.
A tax ID number is not required if you operate a sole proprietorship or an LLC with no employees, in which case you would simply use your own Social Security Number as a tax ID. But you must obtain an EIN if you are a sole proprietor who files pension or excise tax returns. Even partnerships without non-partner employees are required to have tax ID numbers.
Additionally, there are certain instances where you will need to obtain a new EIN (See "Do You Need a New EIN?" from the IRS for more details):
You may apply for a tax ID online, via telephone, by fax, or through the mail.
Get Tax Help for Your Small Business: Start with a Free Review of Your Questions
Unless you operate a sole proprietorship or an LLC with no employees, you will need a tax ID number for your small business. The best way to learn about this and other aspects of business tax law is to speak with an attorney. Get started today with a free legal evaluation of your tax-related situation.