If you received Letter 531 "Notice of Deficiency" from the IRS, please see the following information from the IRS Letters and Notices Offering an Appeal Opportunity website:
You will get this letter if you owe additional tax or other amounts for the tax year(s) listed in the letter. The letter explains how to dispute the adjustments if you do not agree. If you want to dispute the adjustments without payment, you will have 90 days from the notice date to file a petition with the Tax Court.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.