As we migrate out of "tax season," we at WATAX are getting a better idea of what is pulling non-filers out of the woodwork and into "compliance."
The most common impetus for non-filers to act is the IRS Notice of Deficiency (NOD) which is the final round of IRS letters indicating that the IRS has officially filed a return for you and assessed a balance. The NOD allows you another 90 days to go to Tax Court to dispute the assessment, BUT if you or your tax professional are in touch with the IRS examination's office that processed your return before the NOD was issued, you should be able to file the return thru them (or normal channels) without having to go to Tax Court. These days, 2014 and 2016 tax returns seem to be the most commonly targeted un-filed returns.
If you haven't filed returns in years and are really unlucky, you might also be contacted by a Special Agent from the Criminal division of the IRS (CID). Not filing tax returns can be considered a felony and by CID contacting you, your luck has run out. You most certainly will need the representation of an attorney and should also make plans to file your returns. If you clicked on the link for CID above, you will notice that CID is a rare division of the IRS that is hiring!