U.S. citizens and residents are taxed on their income from all sources worldwide.  Worldwide taxation by the U.S. does not disarm the taxing power of other countries.  Americans pursuing income outside of the U.S. are bound to encounter tax collectors asserting their own national claims.  The world is awash in possibilities of double taxation.  Check out my October 14, 2013 blog post for a hypothetical illustrating international double taxation and its main cause: inconsistent sourcing rules in different countries imposing overlapping taxes.