You must file a federal tax return if your income is above a certain level, which varies depending on your age, filing status and the type of income you receive. A married couple under 65 generally does not have to file if their joint income is less than roughly $16,900. However, you must file if you have self-employment income exceeding $400.

Even if you fall below the income thresholds, you might want or need to file a tax return if you had federal income taxes withheld and are entitled to a refund, sold a house, or can claim the earned income tax credit.

For more information, see http://www.irs.gov/individuals/article/0,,id=96623,00.html or this article on the topic http://www.smartmoney.com/tax/advice/index.cfm?story=whennottofile.