Interesting... because someone told me not too long ago that I don't have to pay federal taxes because it's 'voluntary'. Already knowing this couldn't be further from the truth, I did some research to see how anyone could get this idea. Apparently the manner in which you are able to calculate and pay your own taxes (instead of the gov doing it for you) is the voluntary part. But obviously if you get audited and the IRS determines you've underpaid, it is mandatory to pay whatever additional tax is deemed owed. So doesn't seem like there's anything 'voluntary' about it... appears to be just a poor choice of words in the tax code.