Dominant global reserve currencies

U.S. dollar

Worldwide use of the euro and the U.S. dollar
  United States
  External adopters of the US dollar
  Currencies pegged to the US dollar
  Currencies pegged to the US dollar w/ narrow band
  Eurozone
  External adopters of the euro
  Currencies pegged to the euro
  Currencies pegged to the euro w/ narrow band
Note that the Belarusian ruble is pegged to the Euro, Russian Ruble and U.S. Dollar in a currency basket.
In the period following the Bretton Woods Conference of 1944, exchange rates around the world were pegged against the United States dollar, which could be exchanged for a fixed amount of gold. This reinforced the dominance of the US dollar as a global currency.
Since the collapse of the fixed exchange rate regime and the gold standard and the institution of floating exchange rates following the Smithsonian Agreement in 1971, most currencies around the world have no longer been pegged against the United States dollar. However, as the United States remained the world's preeminent economic superpower, most international transactions continued to be conducted with the United States dollar, and it has remained the de facto world currency.
Only two serious challengers to the status of the United States dollar as a world currency have arisen. During the 1980s, the Japanese yen became increasingly used as an international currency,[citation needed] but that usage diminished with the Japanese recession in the 1990s. More recently, the euro has increasingly competed with the United States dollar in international finance.
Since the mid-20th century, the de facto world currency has been the United States dollar. According to Robert Gilpin in Global Political Economy: Understanding the International Economic Order (2001): "Somewhere between 40 and 60 percent of international financial transactions are denominated in dollars. For decades the dollar has also been the world's principal reserve currency; in 1996, the dollar accounted for approximately two-thirds of the world's foreign exchange reserves" (255).

Many of the world's currencies are pegged against the dollar. Some countries, such as Ecuador, El Salvador, and Panama, have gone even further and eliminated their own currency (see dollarization) in favor of the United States dollar. The dollar continues to dominate global currency reserves, with 63.9% held in dollars, as compared to 26.5% held in euros (see Reserve Currency).