On the return you filed, it is your responsibility to declare all income and compute, based on that income, any deductions, credits or other offsets of income, and the tax tables provided to you, exactly what your tax bill is, and then you pay it. The IRS does not bill you, per se. If their figures show that you have either made an error, or underreported income, that results in you owing more, then they will notify you that you must pay the unpaid tax, and probably interest and penalties on top of it. It can take them several years to catch up, though. If your return was done properly and you paid all tax owed based on the return, there's nothing to worry about.