Keynesian economics, developed by John Maynard Keynes in the 1930s, emphasizes government intervention to manage demand and stabilize economies. It argues that aggregate demand drives output and employment, not supply. Key concepts include fiscal policy, the multiplier effect, and the rejection of Say's Law. Keynesian policies dominated post-WWII but faced challenges during the 1970s stagflation. Despite criticisms, it remains central to macroeconomic policy, advocating countercyclical measures.