Unless fraud, evasion, or a substantial understatement of income is involved in your tax return, Uncle Sam generally has only three years to tap you on the shoulder and ask to see the underlying documents necessary to support information reported in your tax return -- a pleasant process known as an audit.

Remember, unlike the "innocent until proven guilty" assumption used by our criminal justice system, with the IRS you must prove the validity of your tax return. You have to sweat out three years before you can rest easy that your return hasn't been selected for audit. Usually that countdown period begins on the date the tax return is required to be filed (April 15th). If you file after your normal filing date, the three-year clock begins to tick on the date that the IRS actually receives your return.

This three-year period is commonly called the "statute of limitations." (Don't confuse it with the statue of limitations -- that's under a few pigeons somewhere in New Jersey and has nothing to do with taxes.) In some cases, the statute of limitations can extend for a longer period of time, but normally you're looking at three years.

As an example, your year 2000 individual income tax return will be due on April 16, 2001 (because April 15 falls on a Sunday). Even if you file your tax return on January 25, 2001 (or any other date prior to April 16), your three-year statute of limitations clock will begin to run on April 16th. This means that your statute period for the 2000 return will expire on April 16, 2004. If you decided to "extend" the due date of your tax return by submitting an automatic extension form, you have also extended your statute of limitations. So, if you file your return on June 20, 2001, your statute will not expire until June 20, 2004.