It is paper currency that is backed up by absolutely nothing, but the rest of the world has been using it to trade with one another and so there is tremendous global demand for our dollars.  The linchpin of this system is the petrodollar.  For decades, if you have wanted to buy oil virtually anywhere in the world you have had to do so with U.S. dollars.  But if one of the biggest oil exporters on the planet, such as Saudi Arabia, decided to start accepting other currencies as payment for oil, the petrodollar monopoly would disintegrate very rapidly. For years, everyone assumed that nothing like that would happen any time soon, but now Saudi officials are warning of a "major shift" in relations with the United States.  In fact, the Saudis are so upset at the Obama administration that "all options" are reportedly "on the table".