The United States dollar (symbol: $; code: USD; also abbreviated US$ or U.S. Dollar, to distinguish it from other dollar-denominated currencies; referred to as the dollar, U.S. dollar, American dollar, or colloquially buck) is the official currency of the United States and several other countries. The Coinage Act of 1792 introduced the U.S. dollar at par with the Spanish silver dollar, divided it into 100 cents, and authorized the minting of coins denominated in dollars and cents. U.S. banknotes are issued in the form of Federal Reserve Notes, popularly called greenbacks due to their predominantly green color.

The monetary policy of the United States is conducted by the Federal Reserve System, which acts as the nation's central bank.

The U.S. dollar was originally defined under a bimetallic standard of 371.25 grains (24.057 g) (0.7735 troy ounces) fine silver or, from 1837, 23.22 grains (1.505 g) fine gold, or $20.67 per troy ounce. The Gold Standard Act of 1900 linked the dollar solely to gold. From 1934, its equivalence to gold was revised to $35 per troy ounce. Since 1971 all links to gold have been repealed.The U.S. dollar became an important international reserve currency after the First World War, and displaced the pound sterling as the world's primary reserve currency by the Bretton Woods Agreement towards the end of the Second World War. The dollar is the most widely used currency in international transactions, and a free-floating currency. It is also the official currency in several countries and the de facto currency in many others, with Federal Reserve Notes (and, in a few cases, U.S. coins) used in circulation.

As of February 10, 2021, currency in circulation amounted to US$2.10 trillion, $2.05 trillion of which is in Federal Reserve Notes (the remaining $50 billion is in the form of coins and older-style United States Notes).

A gold standard is a monetary system in which the standard economic unit of account is based on a fixed quantity of gold. The gold standard was the basis for the international monetary system from the 1870s to the early 1920s, and from the late 1920s to 1932 as well as from 1944 until 1971 when the United States unilaterally terminated convertibility of the US dollar to gold foreign central banks, effectively ending the Bretton Woods system. Many states still hold substantial gold reserves.Historically, the silver standard and bimetallism have been more common than the gold standard. The shift to an international monetary system based on a gold standard reflected accident, network externalities, and path dependence. Great Britain accidentally adopted a de facto gold standard in 1717 when Sir Isaac Newton, then-master of the Royal Mint, set the exchange rate of silver to gold too low, thus causing silver coins to go out of circulation. As Great Britain became the world's leading financial and commercial power in the 19th century, other states increasingly adopted Britain's monetary system.The gold standard was largely abandoned during the Great Depression before being re-instated in a limited form as part of the post-World War II Bretton Woods system. The gold standard was abandoned due to its propensity for volatility, as well as the constraints it imposed on governments: by retaining a fixed exchange rate, governments were hamstrung in engaging in expansionary policies to, for example, reduce unemployment during economic recessions. There is a consensus among economists that a return to the gold standard would not be beneficial, and most economic historians reject the idea that the gold standard "was effective in stabilizing prices and moderating business-cycle fluctuations during the nineteenth century."