Bimetallism, also known as the bimetallic standard, is a monetary standard in which the value of the monetary unit is defined as equivalent to certain quantities of two metals, creating a fixed rate of exchange between them. In all known historical cases, the metals are gold and silver.

For scholarly purposes, "proper" bimetallism is sometimes distinguished as permitting that both gold and silver money are legal tender in unlimited amounts and that gold and silver may be taken to be coined by the government mints in unlimited quantities. This distinguishes it from "limping standard" bimetallism, where both gold and silver are legal tender but only one is freely coined (e.g. the monies of France, Germany, and the United States after 1873), and from "trade" bimetallism, where both metals are freely coined but only one is legal tender and the other is used as "trade money" (e.g. most monies in western Europe from the 13th to 18th centuries). Economists also distinguish legal bimetallism, where the law guarantees these conditions, and de facto bimetallism, where gold and silver coins circulate at a fixed rate.

During the 19th century there was a great deal of scholarly debate and political controversy regarding the use of bimetallism in place of a gold standard or silver standard (monometallism). Bimetallism was intended to increase the supply of money, stabilize prices, and facilitate setting exchange rates. Some scholars argued that bimetallism was inherently unstable owing to Gresham's law, and that its replacement by a monometallic standard was inevitable. Other scholars claimed that in practice bimetallism had a stabilizing effect on economies. The controversy became largely moot after technological progress and the South African and Klondike Gold Rushes increased the supply of gold in circulation at the end of the century, ending most of the political pressure for greater use of silver. It became completely academic after the 1971 Nixon shock; since then, all of the world's currencies have operated as more or less freely floating fiat money, unconnected to the value of silver or gold. Nonetheless, academics continue to debate, inconclusively, the relative use of the metallic standards.

From the 7th century BCE, Asia Minor, especially in the areas of Lydia and Ionia, is known to have created a coinage based on electrum, a natural occurring material that is a variable mix of gold and silver (with about 54% gold and 44% silver). Before Croesus, his father Alyattes had already started to mint various types of non-standardized electrum coins. They were in use in Lydia and surrounding areas for about 80 years. The unpredictability of its composition implied that it had a variable value which was very hard to determine, which greatly hampered its development.

Croesus (reigned c. 560–c. 546 BCE), king of Lydia, who became associated with great wealth. Croesus is credited with issuing the Croeseid, the first true gold coins with a standardised purity for general circulation,

Herodotus mentioned the innovation made by the Lydians:

"So far as we have any knowledge, they [the Lydians] were the first people to introduce the use of gold and silver coins, and the first who sold goods by retail"

— Herodotus, I94