If a person doesn't file taxes, but doesn't owe the IRS additional money, what are the chances of a problem?

Yes, she is required to file based on IRS requirements, but if she doesn't owe money will they really come after her?

They'll probably send a letter....if the IRS computer calculates that she has a refund based on information they have from W2's, they may drop it at that.

Why would she not file? 1) she's required to, from what you said, and 2) the odds are slim that her tax due is exactly zero. She either owes money that she should pay, or she should file to get the refund due to her.

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And there are no penalties for filed returns that have a refund, unless the tax itself was paid after April 15th, and the return filed even later.

There is an income requirement when filing taxes. For example, a single person with no dependents has to file taxes when income exceeds $8,750. So let's say that you made less than that, you would not have to file. Now, here is something that some people do not realize. Even thou some people may not be required to file, you have to file a return to get your tax withholdings back, plus possible earned income credit. Also, for 2007, you have to file in order to get a rebate check. So why wouldn't a person not file? If a person is required to file, they better file. The IRS may be slow but they will eventually catch up.

Owe or don't owe IRS doesn't know that. A tax return must be filed, if not in a couple of years a letter (ASFR) will be sent out telling you that if you don't file IRS will file for you and you will owe, because they will file the return As Single, 1 exemption and standard deduction.
So if money is owed or not file anyway even if it is a $0 return.

Well, they'll apply penalties (and then interest on the penalties) until she files. These will be taken out of any return she was due to receive.

If she has filed in the past, the chances are about 100% that they'll at least send her a letter saying that they're aware she hasn't filed, and the letter will list out what the penalties and interest rates will be.

What would be the point of not filing, if she doesn't owe?

yes. even if you do not owe money, you are required to file and if you don't file, they will catch you and fine you.
when i was younger and more stupid, i did not file on year, since the irs owed me 40 bucks and if figured, "whatever". two years later, they sent me a letter and fined me 100 bucks for failure to file. no interest, since i did not owe anything, but a penalty that was greater than my refund.
sounds messed up and it is, but it is the way they do it.

Usually not. If she sold stocks or something like that, there might be a lot of paperwork to explain why she doesn't owe anything.

Other than that, while technically she's required to file, they aren't likely to come after her.

There may be a proposed assessment based on reported income with tax computed at single rates and standard deduction. This can produce a bogus balance due which will have to be dealt with one way or another.

File.