An IRS notice is an official letter from the IRS informing you of a change to your account, a discrepancy, or a balance due. Common notices include CP2000 (proposed changes based on information mismatches) and CP501 (balance due reminder). Don't panic — most notices are routine and don't mean you're in trouble. Read the notice carefully to understand what the IRS is asking and by when. Ignoring an IRS notice will make the situation worse; always respond by the deadline.