An assessment is the IRS's formal entry that you owe - and the system contains more doors for challenging one than almost anyone knows. Which door is open depends on where you are in the timeline, so here is the complete map, in order of preference.
Before Assessment: The Deficiency Doors
Most disputed assessments must be preceded by a Notice of Deficiency, and the 90-day Tax Court petition is the premier door: the dispute heard before payment, with most petitioned cases settling at Appeals under litigation-risk pricing. Before the 90-day letter, the 30-day letter opens the Appeals protest - same settlement territory, lower stakes. These doors close on their deadlines forever, which is why the entire discipline of IRS mail is reading dates before content.
After Assessment: The Reconsideration Doors
Missed the deficiency doors? Audit reconsideration reopens examined years when you present information the exam never considered - the standard repair for audits that ran without you, mail that never arrived, and documentation found late. Substitute-for-return assessments get the same treatment through filing the actual return, with corrections that routinely collapse inflated balances. And the CDP liability door: a collection due process hearing can reach the underlying liability itself where you never received the deficiency notice or had a prior opportunity to dispute - resurrecting the challenge collection was supposed to foreclose.
The Payment Doors
The classic post-assessment route: pay the disputed tax, file a refund claim, and sue in district court or the Court of Federal Claims if denied - full judicial review at the price of paying first, and for divisible taxes like the trust fund penalty, paying a single quarter's slice of one employee's withholding opens the courthouse without paying the whole assessment. Add the offset doors - doubt-as-to-liability offers, penalty challenges - and the honest summary emerges: almost no assessment is truly final until its collection statute runs. Which door your facts open is a one-conversation diagnosis. Bring me the assessment and its history.