Fractional-reserve banking is the system of banking in all countries worldwide, under which banks that take deposits from the public keep only part of their deposit liabilities in liquid assets as a reserve, typically lending the remainder to borrowers. Bank reserves are held as cash in the bank or as balances in the bank's account at the central bank. Fractional-reserve banking differs from the hypothetical alternative model, full-reserve banking, in which banks would keep all depositor funds on hand as reserves.

The country's central bank may determine a minimum amount that banks must hold in reserves, called the "reserve requirement" or "reserve ratio". Most commercial banks hold more than this minimum amount as excess reserves. Some countries, e.g. the core Anglosphere countries of the United States, the United Kingdom, Canada, Australia, and New Zealand, and the three Scandinavian countries, do not impose reserve requirements at all.

Bank deposits are usually of a relatively short-term duration, and may be "at call" (available on demand), while loans made by banks tend to be longer-term, resulting in a risk that customers may at any time collectively wish to withdraw cash out of their accounts in excess of the bank reserves. The reserves only provide liquidity to cover withdrawals within the normal pattern. Banks and the central bank expect that in normal circumstances only a proportion of deposits will be withdrawn at the same time, and that reserves will be sufficient to meet the demand for cash. However, banks may find themselves in a shortfall situation when depositors wish to withdraw more funds than the reserves held by the bank. In that event, the bank experiencing the liquidity shortfall may borrow short-term funds in the interbank lending market from banks with a surplus. In exceptional situations, such as during an unexpected bank run, the central bank may provide funds to cover the short-term shortfall as lender of last resort.

As banks hold in reserve less than the amount of their deposit liabilities, and because the deposit liabilities are considered money in their own right (see commercial bank money), fractional-reserve banking permits the money supply to grow beyond the amount of the underlying base money originally created by the central bank. In most countries, the central bank (or other monetary policy authority) regulates bank-credit creation, imposing reserve requirements and capital adequacy ratios. This helps ensure that banks remain solvent and have enough funds to meet demand for withdrawals, and can be used to influence the process of money creation in the banking system. However, rather than directly controlling the money supply, contemporary central banks usually pursue an interest-rate target to control bank issuance of credit and the rate of inflation.

Fractional-reserve banking predates the existence of governmental monetary authorities and originated with bankers' realization that generally not all depositors demand payment at the same time. In the past, savers looking to keep their coins and valuables in safekeeping depositories deposited gold and silver at goldsmiths, receiving in exchange a note for their deposit (see Bank of Amsterdam). These notes gained acceptance as a medium of exchange for commercial transactions and thus became an early form of circulating paper money. As the notes were used directly in trade, the goldsmiths observed that people would not usually redeem all their notes at the same time, and they saw the opportunity to invest their coin reserves in interest-bearing loans and bills. This generated income for the goldsmiths but left them with more notes on issue than reserves with which to pay them. A process was started that altered the role of the goldsmiths from passive guardians of bullion, charging fees for safe storage, to interest-paying and interest-earning banks. Thus fractional-reserve banking was born.

If creditors (note holders of gold originally deposited) lost faith in the ability of a bank to pay their notes, however, many would try to redeem their notes at the same time. If, in response, a bank could not raise enough funds by calling in loans or selling bills, the bank would either go into insolvency or default on its notes. Such a situation is called a bank run and caused the demise of many early banks.

These early financial crises led to the creation of central banks. The Swedish Riksbank was the world's first central bank, created in 1668. Many nations followed suit in the late 1600s to establish central banks which were given the legal power to set a reserve requirement, and to specify the form in which such assets (called the monetary base) were required to be held. In order to mitigate the impact of bank failures and financial crises, central banks were also granted the authority to centralize banks' storage of precious metal reserves, thereby facilitating transfer of gold in the event of bank runs, to regulate commercial banks, and to act as lender-of-last-resort if any bank faced a bank run. The emergence of central banks reduced the risk of bank runs – inherent in fractional-reserve banking – and allowed the practice to continue as it does today, as the system of banking prevailing in almost all countries worldwide.

During the twentieth century, the role of the central bank grew to include influencing or managing various macroeconomic policy variables, including measures of inflation, unemployment, and the international balance of payments. In the course of enacting such policy, central banks have from time to time attempted to manage interest rates, reserve requirements, and various measures of the money supply and monetary base.