National central banks play an important role in the forex Exch. market.

They try to control the money supply, inflation, and/or interest rates and often have official or unofficial target rates for their currencies. They can use their often substantial foreign exchange reserves, to stabilize the market. Milton Friedman argued that the best stabilization strategy would be for central banks to buy when the exchange rate is too low, and to sell when the rate is too high - that is, to trade for a profit. Nevertheless, central banks do not go bankrupt if they make large losses, like other traders would, and there is no convincing evidence that they do make a profit trading.The mere expectation or rumor of central bank intervention might be enough to stabilize a currency, but aggressive intervention might be used several times each year in countries with a dirty float currency regime. Central banks do not always achieve their objectives, however. The combined resources of the market can easily overwhelm any central bank.

The main trading centers are in London, New York, and Tokyo, but banks throughout the world participate. As the Asian trading session ends, the European session begins, then the US session, and then the Asian begin in their turns. Traders can react to news when it breaks, rather than waiting for the market to open.
In April 2004 the average daily international foreign exchange trading volume was $1.9 trillion, according to the BIS study (Triennial Central Bank Survey of Foreign Exchange and Derivatives Market Activity 2004 - Final Results).