The income tax in the United States is a voluntary system. However, this does not mean that paying income tax is optional. The federal government expects every taxpayer voluntarily to report all their income in the tax return and calculate the corresponding tax using the tax laws. The IRS (Internal Revenue Service), the service in charge of collecting the income taxes in the United States has put security measures in place to ensure that its residents pay the correct amount. For example, the agency receives a copy of your W-2 form each year, and if you do not file a statement they can easily calculate the tax and send you a bill.