A letter from the Internal Revenue Service that advises a taxpayer about delinquent taxes owed plus any penalties and interest. A notice of deficiency is sometimes also called a 90-day letter because it gives the taxpayer 90 days to dispute the tax assessment. If the taxpayer disagrees with the assessment, he or she can file an appeal with the U.S. Tax Court.

Sometimes a taxpayer can resolve a dispute with the IRS without going to court by going through an appeal process. The Request for Appeals Review form asks the taxpayer to list each item of the IRS decision that is in dispute and why. Taxpayers can represent themselves during the appeals process or hire a certified public accountant, attorney or enrolled agent to represent them. The IRS does not allow taxpayers to disagree based on moral, religious, political, constitutional or conscientious objection grounds.