A reserve requirement is a central bank regulation that sets the minimum amount that a commercial bank must hold in liquid assets. This minimum amount, commonly referred to as the commercial bank's reserve, is generally determined by the central bank on the basis of a specified proportion of deposit liabilities of the bank. This rate is commonly referred to as the reserve ratio. Though the definitions vary, the commercial bank's reserves normally consist of cash held by the bank and stored physically in the bank vault (vault cash), plus the amount of the bank's balance in that bank's account with the central bank. A bank is at liberty to hold in reserve sums above this minimum requirement, commonly referred to as excess reserves. This measure is commonly referred to as the liquidity ratio.

The reserve ratio is sometimes used by a country’s monetary authority as a tool in monetary policy, to influence the country's money supply by limiting or expanding the amount of lending by the banks. Monetary authorities increase the reserve requirement only after careful consideration because an abrupt change may cause liquidity problems for banks with low excess reserves; they generally prefer to use open market operations (buying and selling government-issued bonds) to implement their monetary policy. In the United States and many other countries (except Brazil, China, India, Russia), reserve requirements are generally not altered frequently in implementing a country's monetary policy because of the short-term disruptive effect on financial markets.