From Ron Paul:

We frequently hear the financial press refer to the U.S. dollar as the "world's reserve currency," implying that our dollar will always retain its value in an ever shifting world economy. But this is a dangerous and mistaken assumption.

Since August 15, 1971, when President Nixon closed the gold window and refused to pay out any of our remaining 280 million ounces of gold, the U.S. dollar has operated as a pure fiat currency. This means the dollar became an article of faith in the continued stability and might of the U.S. government.

In essence, we declared our insolvency in 1971. Everyone recognized some other monetary system had to be devised…

[link to paul.house.gov]