An Employer Identification Number is a nine-digit number issued by the IRS. It is similar to a Social Security number, and is used by your employer at tax time to report income on tax returns. It's also used in the day-to-day operations, such as with dealings with investors, contractors and other entities with whom your employer does business. You need to have your employer's EIN to report income on your tax return so that the IRS can match up your stated income with the payment the employer reports on its return.
Check your W2 form. There's a box for the EIN: The number should be filled in.
Call your employer's payroll or accounting department and ask for the EIN for your tax form. Once you have been identified as an employee, the department should give you the requested information.
Check your employer's website. The EIN may be there, especially if there are annual reports, invoices or other documents on the site.
Check out the Securities and Exchange Commission's Edgar database if your employer is a corporation. The SEC has 7 million documents (as of May 2010) available for free download. Your employer's EIN will be on any of its SEC filings, usually on the front page.