[영문]The third of three episodes in a major natural experiment in monetary policy that started more than 80 years ago is just now coming to an end. The experiment consists in observing the effect on the economy and the stock market of the monetary policies followed during and after three very similar periods of rapid economic growth in response to rapid technological change: the booms of the 1920s in the United States, the 1980s in Japan and the 1990s in the United States. In this experiment, the quantity of money is the counterpart of the experimenter's input. The performance of the economy and the level of the stock market are the counterpart of the experimenter's output. The results of this natural experiment are clear, at least for major ups and downs: what happens to the quantity of money has a determinative effect on what happens to national income and to stock prices. The results strongly support Anna Schwartz's and my 1963 conjecture about the role of monetary policy in the Great Contraction. They also support the view that monetary policy deserves much credit for the mildness of the recession that followed the collapse of the U.S. boom in late 2000.