The paper was prepared for the 80-th anniversary of publishing of John Maynard Keynes’ “General Theory of Employment, Interest and Money”. It discusses the stages of the economist’s life, the main books written prior to "The General Theory ...". Particular attention is devoted to the development issues of the monetary policy in the works of "Indian Currency and Finance", ”A Tract on Monetary Reform” and "A Treatise on Money". A special section is dedicated to the analysis of Keynes’ methodology, its logic and structure, influenced by John. E. Moore. The paper reveals the unity and the difference in approaches of A. Marshall and John M. Keynes, and explores new categories of behavioral economics and marginal analysis, which established the success of "General Theory of Employment, Interest and Money", shows the value of Keynes's theory for the further development of macroeconomics. Particular attention is paid to the popularization of Keynes's ideas from the initial interpretations of "The General Theory ..." to the neoclassical synthesis and further to neo-Keynesianism and post-Keynesianism. The paper studies the unity and the distinction between Hicks’ and American Keynesianism. Hicksian assumptions of a savings-investment function have determined the features of the IS-LM model. The contributions to the development of Keynesianism A. Hansen and P. A. Samuelson are also shown, as well as the history of the "Keynesian Cross". A comparative analysis of the neoclassical and Keynesian models of general economic equilibrium is given and analyzes the institutional reasons explaining differences between neoclassical and Keynesian paradigms. A special section is devoted to the Keynesian theory of growth, showing unity and difference of R. Harrod and E. Domar models, along with their impact on the creation of Development Economics. Simplified understanding of Keynes's legacy has caused the emergence of unorthodox Keynesianism. The paper focuses on the contributions of R. Clower and A. Leijonhufvud to grasping the role of uncertainty and expectations in the original concepts of Keynes and his contributions to the study of disequilibrium economy. Finally, we investigate the contribution of post-Keynesians (J. V. Robinson, P. Sraffa and N. Kaldor), the rise and the fall of the Phillips curve, as well as the criticism of the Keynesian doctrine given by the monetarists, adherents of supply-side economics, new classical economics, the theory of rational expectations and the public choice.