Keynesian economics ( /ˈkeɪnziən/ KAYN-zee-ən; also called Keynesianism and Keynesian theory) are the group of macroeconomic schools of thought based on the ideas of 20th-century economist John Maynard Keynes.

Advocates of Keynesian economics argue that private sector decisions sometimes lead to inefficient macroeconomic outcomes which require active policy responses by the public sector, particularly monetary policy actions by the central bank and fiscal policy actions by the government to stabilize output over the business cycle. The theories forming the basis of Keynesian economics were first presented in The General Theory of Employment, Interest and Money, published in 1936. The interpretations of Keynes are contentious and several schools of thought claim his legacy.

Keynesian economics advocates a mixed economy — predominantly private sector, but with a significant role of government and public sector — and served as the economic model during the later part of the Great Depression, World War II, and the post-war economic expansion (1945–1973), though it lost some influence following the tax surcharge in 1968 and the stagflation of the 1970s. The advent of the global financial crisis in 2008 has caused a resurgence in Keynesian thought.

John Maynard Keynes, 1st Baron Keynes,CB FBA ( /ˈkeɪnz/ KAYNZ; 5 June 1883–21 April 1946) was a British economist whose ideas have profoundly affected the theory and practice of modern macroeconomics, as well as the economic policies of governments. He greatly refined earlier work on the causes of business cycles, and advocated the use of fiscal and monetary measures to mitigate the adverse effects of economic recessions and depressions. His ideas are the basis for the school of thought known as Keynesian economics, as well as its various offshoots.

In the 1930s, Keynes spearheaded a revolution in economic thinking, overturning the older ideas of neoclassical economics that held that free markets would, in the short to medium term, automatically provide full employment, as long as workers were flexible in their wage demands. Keynes instead argued that aggregate demand determined the overall level of economic activity, and that inadequate aggregate demand could lead to prolonged periods of high unemployment. Following the outbreak of World War II, Keynes's ideas concerning economic policy were adopted by leading Western economies. During the 1950s and 1960s, the success of Keynesian economics resulted in almost all capitalist governments adopting its policy recommendations.