If IRS selects your return for an audit, the audit will generally cover a 3 year period. That is the statute of limitations IRS has to audit a tax return. More specifically, this is 3 years from the date you file your return. If you filed the last 5 years of your tax returns last year, IRS will have a couple more years to open all those returns for an audit.
If you have a “substantial understatement of income” (you fail to report 25% or more of your gross income), IRS can go back 6 years.
If you fail to file a return or file a fraudulent return – there is NO statute of limitations. TO have a valid return if you file a paper return, it needs to be signed by you and, if you are married, by your spouse.