Because they are backed by the U.S. government (meaning that all principal and interest will be paid on time), they are considered extremely safe, as there is very little risk that the U.S. government will default on the loans. In 2013, while the U.S. Congress was deciding how to raise the debt ceiling, there were fears that the U.S. debt ceiling would not be raised, making many holders of U.S. backed securities fearful of default, but America did not default. Since there is a very large global market for U.S. Treasury securities, they can easily be purchased and sold.