What Is a Tax Audit, and Who Actually Conducts One?

Quick answer: A tax audit is a formal review of your tax return where a government agency checks that your reported income, deductions, and credits match your actual financial records. At the federal level, the IRS conducts these reviews, either by mail (a correspondence audit) or in person (a field or office audit). Most audits aren't random — they're triggered by specific red flags on a return, like income that doesn't match employer records or deductions that look unusually large for your income level.

Key takeaways

• The IRS selects returns for audit using a mix of computer scoring, random sampling, and mismatches with third-party records like W-2s and 1099s.
• Most IRS audits are correspondence audits conducted entirely by mail, not the in-person "agent shows up at your door" scenario people picture.
• The IRS generally has three years from your filing date to audit a return, though that window extends to six years if income is substantially understated, according to the IRS statute of limitations rules.
• Getting an audit notice does not mean you did something wrong — it often just means one number on your return needs documentation.

What Does a Tax Audit Actually Check?

A tax audit checks whether the numbers on your filed return can be backed up with real documentation. The auditor isn't guessing whether you were honest — they're comparing your return against records the government already has, like W-2s from employers, 1099s from clients or banks, and mortgage interest statements from lenders.

If those third-party records don't match what you reported, that's usually where the review starts. Auditors typically focus on a narrow set of items rather than re-examining your entire financial life:

• Unreported income that shows up on a 1099 or K-1 but not on your return
• Business deductions that seem large relative to reported revenue
• Home office, vehicle, and travel expense claims
• Charitable contribution deductions without matching receipts
• Credits that require specific eligibility documentation, like the Earned Income Tax Credit

The scope of the audit depends on the type. A correspondence audit might ask you to mail in proof of a single deduction. A full field audit, where an IRS revenue agent reviews your books in person, can cover multiple years and several categories of income and expense at once.

Who Actually Conducts a Tax Audit?

At the federal level, it's the Internal Revenue Service, specifically employees called revenue agents or tax examiners, depending on the audit type. Tax examiners typically handle correspondence audits — the mailed notices asking for documentation on a specific line item. Revenue agents handle more complex field audits, which involve reviewing a business's books, interviewing the taxpayer, and sometimes visiting the business location.

States run their own parallel audit programs. If you live in Illinois, the Illinois Department of Revenue can audit your state return independently of anything the IRS does, and a federal audit adjustment often triggers a matching state review since Illinois taxable income starts from your federal figures.

It's worth being clear about who does not conduct an audit: your tax preparer doesn't decide you're getting audited, and no private company can "flag" your return for IRS review. That decision sits entirely with the IRS's internal selection systems and, occasionally, referrals from other government agencies. If you're unclear on what role a preparer, advisor, or CPA plays in your tax situation versus what the IRS does, we covered those distinctions in Tax Consultant vs Tax Preparer vs CPA: Who Do You Need?

What Triggers an Audit in the First Place?

Most audits start with a mismatch, not a hunch. The IRS uses a scoring system called the Discriminant Information Function (DIF), which compares your return against statistical norms for people in similar income brackets and professions, according to the IRS's public description of its examination process. A return that scores far outside the typical range for its category gets flagged for a closer look.

Beyond the DIF score, specific situations tend to draw more attention:

Trigger	Why it raises a flag
Income mismatch	Employer or client reported income you didn't include on your return
Large charitable deductions	Deduction amount is high relative to reported income
Home office deduction	Frequently misapplied, so it draws routine scrutiny
Cash-heavy business	Harder to verify income independently, so it's checked more closely
Round numbers everywhere	Estimated figures instead of exact records suggest poor recordkeeping
Claiming 100% business use of a vehicle	Rarely accurate and often questioned
Self-employment losses year after year	May look like a hobby rather than a business under IRS rules

None of these guarantee an audit. Plenty of taxpayers claim a home office deduction or run a cash-based business without ever hearing from the IRS. But each one increases the statistical odds that your return gets a second look.

Don't skip this: if you receive an audit notice, respond by the deadline printed on it. Ignoring an IRS notice doesn't make the review go away — it typically leads the IRS to adjust your return on its own, based only on the information it already has, which is rarely in your favor.

How Long Does the IRS Have to Audit a Return?

Generally three years from the date you filed, but that window can stretch to six years or have no limit at all in specific situations. The IRS lays out these rules under its statute of limitations guidance:

• Three years is the standard window for most returns, starting from the filing date or the due date, whichever is later.
• Six years applies if you omitted more than 25% of your gross income from the return.
• No time limit applies if you never filed a return at all, or if the IRS determines the return was fraudulent.

That last point matters more than people expect. If you have a year — or several years — where you simply never filed, there's no clock running in your favor. The return can be examined whenever the IRS gets to it, and unfiled returns are one of the more common reasons people end up dealing with back-tax notices years after the fact. If that describes your situation, it's worth reading through what's involved in filing back taxes before an audit notice forces the issue.

What Happens After an Audit — and What If You Disagree?

An audit ends one of three ways: no change to your return, a change you agree to, or a change you dispute. If the IRS proposes an adjustment and you agree, you sign the examination report, pay any additional tax owed (often with interest and possibly penalties), and the matter closes.

If you disagree, you have documented appeal rights. You can request a conference with the IRS Office of Appeals, which operates independently from the examination division, or in some cases petition the U.S. Tax Court before paying the disputed amount. The IRS explains these options in the audit notice itself, and the specific deadline to respond is printed on that letter — missing it can forfeit your appeal rights, so treat that date as fixed.

Here's a practical checklist if you've just received an audit notice:

• Read the notice fully to identify exactly which tax year and which line items are being questioned
• Note the response deadline and put it on your calendar immediately
• Gather receipts, bank statements, and records tied specifically to the flagged items
• Avoid sending the IRS anything beyond what was requested
• Contact a tax professional before your first response if the audit involves business income, multiple years, or a large proposed adjustment
• Keep copies of everything you send and the envelope's mailing receipt or portal confirmation

Do You Need Help, or Can You Handle It Alone?

A simple correspondence audit asking for one receipt is often manageable on your own — a straightforward mismatch with clear documentation rarely needs representation. But a field audit, a multi-year review, or any audit touching business income is a different situation, since the auditor is trained to look for issues you might not know to protect against.

This is where the difference between preparing a return and advising on one really shows up. A tax preparer files the return; a tax advisor helps you understand the strategy and risk behind the numbers before problems start. We go into that distinction in What Is Tax Advisory, and Do You Need One? — and it's especially relevant once an audit notice is already in your mailbox, because at that point the goal shifts from filing correctly to defending what was already filed.

If you're facing an audit, back taxes, or an IRS penalty notice and aren't sure where to start, USTAXX Consulting Services works with individuals and small businesses across Illinois and nationwide on exactly these situations — from IRS correspondence assistance to full back-tax resolution. Our team, led by Akmammet on the tax preparation side, has built a reputation among clients for translating dense IRS language into a plan you can actually follow, without the stress of guessing what a notice means. Reach out through ustaxx.com if you've received a notice and want a clear read on what it actually requires from you.

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