Tax law is so complicated that nearly everyone makes a mistake sooner or later. What should a taxpayer do when he or she learns of a mistake on a previously filed tax return? Of course, if the taxpayer overpaid her tax, she probably will jump at the opportunity to amend the return and claim a refund. However, if the prior return underreported tax, is the taxpayer required to file an amended return to correct the understatement? And what are the consequences of filing an amended return? How should a tax practitioner advise a client? This column addresses when and how a taxpayer can or should file amended returns and how tax practitioners should approach issues relating to a taxpayer’s prior non-compliance.