Keynesian economics is an economic theory named after John Maynard Keynes (1883 - 1946), a British economist. It was his simple explanation for the cause of the Great Depression for which he is most well-known.

His ideas spawned a slew of interventionist economic policies during the Great Depression. Keynes' economic theory was based on an circular flow of money. One person's spendings goes towards anothers earnings, and when that person spends her earnings she is, in effect, supporting anothers earnings. This circle continues on and helps support a normal functioning economy. When the Great Depression hit, people's natural reaction was to hoard their money. However, under Keynes' theory this stopped the circular flow of money, keeping the economy at a standstill.

Keynes' solution to this poor economic state was to prime the pump. By prime the pump, Keynes argued that the government should step in to increase spending, either by increasing the money supply or by actually buying things on the market itself. In the times of the Great Depression, however, this was an understandably unpopular solution. It is said, however, that the massive defense spending that United States President Franklin Delano Roosevelt initiated helped cure the US economy.

Since Keynesian economics advocates for the public sector to step in to assist the economy generally, it is a significant departure from popular economic thought which preceded it — laissez-fair capitalism. Laissez-fair capitalism supported the exclusion of the public sector in the market. The belief was that an unfettered market would achieve balance on its own. Proponents of free-market capitalism include the Austrian School of economic thought, of which one of its earliest founders, Friedrich von Hayek, also lived in England alongside Keynes. The two had a public rivalry for many years because of their opposing thoughts on the role of the state in the economic lives of individuals.

Keynesian economics warns against the practice of too much saving (underconsumption) and not enough consumption (spending) in the economy, and it also supports considerable redistribution of wealth, when needed. Keynesian economics further concludes that there is a pragmatic reason for the massive redistribution of wealth: if the poorer segments of society are given sums of money, they will likely spend it, rather than save it, thus promoting economic growth. Another central idea of Keynesian economics is that trends in the macroeconomic level can disproportionately influence consumer behavior at the micro-level. Keynesian economics, also called macroeconomics for it's macro look at the economy, remains one of the important schools in economic thought today.