Who must file US Federal tax returns?
U.S. citizens and resident aliens (e.g. Green Card holders) are taxed on their world-wide income regardless of where it is earned, paid, or received. But by living abroad you get certain tax benefits that are not available in the U.S. Nonresident aliens (e.g. an individual without permanent residency in U.S.) are subject to tax only on their US source income.
