Don't panic, but don't set it aside either
The overwhelming majority of IRS notices are computer-generated and routine — a mismatch between what you reported and what a third party (an employer, a bank, a broker) reported, a math correction, or a request for a document that's missing from your file. They are not, on their own, an audit. But every notice has a response deadline printed on it, usually 30 days, and the response options narrow considerably once that date passes.
Work out what kind of letter you actually have
The notice number, usually in the top corner, tells you what you're dealing with. A CP2000 means the IRS's records don't match your return — commonly a 1099 or W-2 you or your preparer missed. A CP14 is a balance-due notice. Letters with a name like an audit notice or examination letter are a different and more serious category, and those especially should not be handled alone.
Read the whole thing before reacting to the first number you see. Notices sometimes propose a balance that's wrong, and the letter itself explains how to dispute it — but only within its deadline.
What to do in the first 48 hours
Confirm it's real: the IRS does not initiate contact by email, text, or a phone call demanding immediate payment. If you got one of those instead of a letter, it's very likely a scam. For an actual mailed notice, make a copy, note the response deadline on a calendar, and gather anything the letter references — the return in question, the document it says doesn't match. Do not send a payment or sign anything before you understand what the notice is actually claiming.
When to call a professional
A straightforward math correction you agree with is something many people handle themselves. Anything involving a dollar figure you don't recognize, a business return, more than one tax year, or the word 'examination' is worth a call before you respond. A CPA or enrolled agent with power of attorney can talk to the IRS directly on your behalf — which, for most people, is the single most stressful part of the process removed entirely.
What resolution looks like
Most routine notices end one of three ways: you agree and pay or arrange a payment plan, you dispute it with documentation and the IRS revises or drops it, or it turns out to be a processing error that resolves once corrected. Whichever it is, get written confirmation that the matter is closed — a closing letter or an updated notice showing a zero balance — and keep it with that year's tax file indefinitely.
This is general information, not tax advice.