The quantity of foreign exchange reserves can change as a central bank implements monetary policy.[1] A central bank that implements a fixed exchange rate policy may face a situation where supply and demand would tend to push the value of the currency lower or higher (an increase in demand for the currency would tend to push its value higher, and a decrease lower). In a flexible exchange rate regime, these operations occur automatically, with the central bank clearing any excess demand or supply by purchasing or selling the foreign currency. Mixed exchange rate regimes ('dirty floats', target bands or similar variations) may require the use of foreign exchange operations (sterilized or unsterilized[clarification needed]) to maintain the targeted exchange rate within the prescribed limits .
Foreign exchange operations that are unsterilized will cause an expansion or contraction in the amount of domestic currency in circulation, and hence directly affect monetary policy and inflation: An exchange rate target cannot be independent of an inflation target. Countries that do not target a specific exchange rate are said to have a floating exchange rate, and allow the market to set the exchange rate; for countries with floating exchange rates, other instruments of monetary policy are generally preferred and they may limit the type and amount of foreign exchange interventions. Even those central banks that strictly limit foreign exchange interventions, however, often recognize that currency markets can be volatile and may intervene to counter disruptive short-term movements.
To maintain the same exchange rate if there is increased demand, the central bank can issue more of the domestic currency and purchase the foreign currency, which will increase the sum of foreign reserves. In this case, the currency's value is being held down; since (if there is no sterilization) the domestic money supply is increasing (money is being 'printed'), this may provoke domestic inflation (the value of the domestic currency falls relative to the value of goods and services).
Since the amount of foreign reserves available to defend a weak currency (a currency in low demand) is limited, a foreign exchange crisis or devaluation could be the end result. For a currency in very high and rising demand, foreign exchange reserves can theoretically be continuously accumulated, although eventually the increased domestic money supply will result in inflation and reduce the demand for the domestic currency (as its value relative to goods and services falls).