Who must file a tax return?

In 2009, more than 141 million individual tax returns were filed with the IRS -- representing just under half of all households in the U.S. Clearly, not all households or people believe they must file a federal income tax return. So, who exactly has to file a return?

If you're a U.S. citizen or resident, that depends on three factors: (1) gross income; (2) filing status, and (3) age.

Gross income is defined in the Tax Code as "all income from whatever source derived" unless otherwise exempt. That sounds complicated, but more or less, think of your gross income as everything you receive before deductions.