As of 2014, the IRS can audit tax returns that have been filed within the past three years. However, if a substantial error is found, the agency can include additional years. In this case, an audit usually does not cover returns that date back further than the past six years.
Generally, the IRS attempts to conduct audits as soon as possible after tax returns are filed. Most audits pertain to tax returns filed during the past two years.
Audits can be conducted entirely by mail or may be conducted in person at a local IRS office. If the audit does not result in a timely resolution, the IRS may request an extension on the statute of limitations for assessing a return.