The IRS is not likely to come and arrest you if you do not file or pay by the due dates, but they don’t like you to be late.

There are several penalties that accrue when you’re late:

1) Late filing penalties vary according to what type of return is being filed. Some are based on a percentage of tax owed, others on the number of days or months you’re late in filing. Partnership returns have a penalty per partner – the more partners, the higher the penalty.

2) Late payment penalties are based on a percentage of the tax owed multiplied by the number of days or months that you’re late in making the payment. Remember, even if you’ve managed to put your return on extension, you’re responsible to make all payments of tax for the prior year by April 15th of the current year.

3) Late payment penalties on estimated tax payments are incurred when you didn’t pay in enough tax over the course of the year, either through withholdings or through estimated tax payments. Remember, our tax system is a “pay-as-you-go” model; you’re supposed to be paying in the taxes you owe as you earn the money. If you can’t do that, you should at least be paying in as much as your total tax liability was for the prior year (“Safe Harbor”).

Keep in mind that, just because the IRS can assess a penalty, it doesn’t mean that it’s set in stone. If you have a really good reason why you missed a deadline, you can ask that penalties be waived for cause, such as an illness or a natural disaster. In fact, in the case of natural disasters, the IRS often automatically extends the deadlines for everyone located in the disaster area.