The United States federal income tax system is a pay-as-you-go operation. If you're an employee, your employer withholds tax money from your paycheck. If you're self-employed, you typically have to pay your taxes on a quarterly basis. You must file a federal income tax return if your income exceeds the minimum for your filing status, but you can file a return even if you have no income at all.
You don't have to file a federal income tax return unless your income exceeds minimum levels mandated by federal tax law. These levels vary based on your age and filing status and might be adjusted by Congress from time to time. For the 2011 tax year, you had to file a tax return if you and your spouse had a combined gross income of $19,000 or more and you filed a joint return. The minimum threshold decreases significantly to $3,700 if the two of you file separate returns.
The Internal Revenue Service has different minimum income levels based on your marital status, age, eyesight and type of income. For example, if you're married, under age 65 and not blind, you have to file a federal income tax return even if you only had unearned income of at least $950. If you filed separate returns and your spouse itemized his deductions, you would have to file a tax return if you earned $5 or more.
If you're the independent type working for yourself, you have to file a return even if you made no more than $400 in the tax year. On the other end of things, if you can be claimed as another person's dependent, you also have to file a tax return if you owe taxes. You have to file if your employer failed to withhold taxes regardless of how much money you earned. The IRS is also expecting a return if you owe Social Security or Medicare taxes on unreported tips.
Filing Below Minimums
If you earn less than the IRS's required minimums, you don't need to file a federal income tax return. However, you might want to file anyway. If your employer withheld taxes, you might have a refund coming, and you can only claim it if you file a return. Some types of tax credits, such as the additional child tax credit, are refundable. If they are more than what you owe, you get the money back as a tax refund.