A currency crisis, which is also called a balance-of-payments crisis, is a sudden devaluation of a currency caused by chronic balance-of-payments deficits which usually ends in a speculative attack in the foreign exchange market. It occurs when the value of a currency changes quickly, undermining its ability to serve as a medium of exchange or a store of value. Currency crises usually affect fixed exchange rate regimes, rather than floating regimes.
A currency crisis is a type of financial crisis, and is often associated with a real economic crisis. Currency crises can be especially destructive to small open economies or bigger, but not sufficiently stable ones. Governments often take on the role of fending off such attacks by satisfying the excess demand for a given currency using the country's own currency reserves or its foreign reserves (usually in the United States dollar, Euro or Pound sterling).