Currency crises in the 1990s, especially those in emerging markets, have sharply disrupted economic activity, affecting not only the country experiencing the crisis, but also those with trade, investment, and geographic links. The authors review the theoretical literature and empirical evidence regarding these crises. They conclude that their primary cause is a fixed nominal exchange rate combined with macroeconomic imbalances, such as current account or fiscal deficits, that the market perceives as unsustainable at the prevailing real exchange rate. They also conclude that currency crises can be prevented through the adoption of sound monetary and fiscal policies, effective regulation and supervision of the financial sector, and a more flexible nominal exchange rate.