This paper compares Marx's economics with those by Sraffa, Keynes, Kalecki and Minsky. The paper takes an ‘ex post’ view on the matter and looks at the output side of the respective authors, but not at the input side. This means no attempt is made to study in a systematic way whether and to what extent Sraffa, Keynes, Kalecki and Minsky were individually influenced by Marx's work. First, the relationship between Marx's theory of value and Sraffa's reformulation of the classical theory of prices and distribution is reviewed. Then the relationship between Marx's and Keynes's monetary theory is examined, relying on an interpretation of Marx's theory of value as a ‘monetary theory of value’. Next, some light is shed on the Marx–Kalecki connection, focusing on Marx's theory of simple and extended reproduction and the built-in, although not fully elaborated, ‘principle of effective demand’ and the related theories of distribution and accumulation. Finally, Marx's and Minsky's views on financial instability and crises are scrutinised. It is concluded that Marx should not be considered as an ‘early post-Keynesian’ but rather as an important forerunner of modern post-Keynesianism, with certain similarities, but also some important differences, and several areas of compatibility.