An open market operation is the sale or purchase of mostly government securities, in a market open to private investors, by a central bank. Sales in the open market are an integral part of monetary policy, allowing the central bank to manage the volume of money and credit in the economy. For instance, if the money supply rises due to an unanticipated in-flow of foreign currency, the central bank may act to counter the rise in foreign exchange reserves by selling government securities; an action referred to as (monetary) sterilisation.