I just realized that I must file U.S. income tax returns for prior years. How many years back do I have to file?

You must file a federal income tax return for any tax year in which your gross income is equal to or greater than the personal exemption amount and standard deduction combined (per the Form 1040 Instructions for the corresponding tax year). Generally, you need to file returns going back six years. This will depend on the facts and circumstances of your particular situation.