Monetary policy is the policy adopted by the monetary authority of a nation to affect monetary and other financial conditions to accomplish broader objectives like high employment and price stability (normally interpreted as a low and stable rate of inflation). Further purposes of a monetary policy may be to contribute to economic stability or to maintain predictable exchange rates with other currencies. Today most central banks in developed countries conduct their monetary policy within an inflation targeting framework, whereas the monetary policies of most developing countries' central banks target some kind of a fixed exchange rate system. A third monetary policy strategy, targeting the money supply, was widely followed during the 1980s, but has diminished in popularity since then, though it is still the official strategy in a number of emerging economies.

The tools of monetary policy vary from central bank to central bank, depending on the country's stage of development, institutional structure, tradition and political system. Interest-rate targeting is generally the primary tool, being obtained either directly via administratively changing the central bank's own interest rates or indirectly via open market operations. Interest rates affect general economic activity and consequently employment and inflation via a number of different channels, known collectively as the monetary transmission mechanism, and are also an important determinant of the exchange rate. Other policy tools include communication strategies like forward guidance and in some countries the setting of reserve requirements. Monetary policy is often referred to as being either expansionary (lowering rates, stimulating economic activity and consequently employment and inflation) or contractionary (dampening economic activity, hence decreasing employment and inflation).

Monetary policy affects the economy through financial channels like interest rates, exchange rates and prices of financial assets. This is in contrast to fiscal policy, which relies on changes in taxation and government spending as methods for a government to manage business cycle phenomena such as recessions. In developed countries, monetary policy is generally formed separately from fiscal policy, modern central banks in developed economies being independent of direct government control and directives.

How best to conduct monetary policy is an active and debated research area, drawing on fields like monetary economics as well as other subfields within macroeconomics.

Monetary policy has evolved over the centuries, along with the development of a money economy. Historians, economists, anthropologists and numismatics do not agree on the origins of money. In the West the common point of view is that coins were first used in ancient Lydia in the 8th century BCE, whereas some date the origins to ancient China. The earliest predecessors to monetary policy seem to be those of debasement, where the government would melt coins down and mix them with cheaper metals. The practice was widespread in the late Roman Empire, but reached its perfection in western Europe in the late Middle Ages.

For many centuries there were only two forms of monetary policy: altering coinage or the printing of paper money. Interest rates, while now thought of as part of monetary authority, were not generally coordinated with the other forms of monetary policy during this time. Monetary policy was considered as an executive decision, and was generally implemented by the authority with seigniorage (the power to coin). With the advent of larger trading networks came the ability to define the currency value in terms of gold or silver, and the price of the local currency in terms of foreign currencies. This official price could be enforced by law, even if it varied from the market price.

Paper money originated from promissory notes termed "jiaozi" in 7th-century China. Jiaozi did not replace metallic currency, and were used alongside the copper coins. The succeeding Yuan dynasty was the first government to use paper currency as the predominant circulating medium. In the later course of the dynasty, facing massive shortages of specie to fund war and maintain their rule, they began printing paper money without restrictions, resulting in hyperinflation.

With the creation of the Bank of England in 1694, which was granted the authority to print notes backed by gold, the idea of monetary policy as independent of executive action^[how?] began to be established. The purpose of monetary policy was to maintain the value of the coinage, print notes which would trade at par to specie, and prevent coins from leaving circulation. During the period 1870–1920, the industrialized nations established central banking systems, with one of the last being the Federal Reserve in 1913. By this time the role of the central bank as the "lender of last resort" was established. It was also increasingly understood that interest rates had an effect on the entire economy, in no small part because of appreciation for the marginal revolution in economics, which demonstrated that people would change their decisions based on changes in their opportunity costs.