"Dynamic economics" is concerned with change and readjustment in economic life. A distinctive doctrine of the present book is that the great bulk of exchanging grows out of dynamic change, and that speculation, in particular, constitutes by far the major part of all trade. From this it follows that the main work of money and credit, as instruments of exchange, is done in the process of dynamic readjustment, and, consequently, that the theory of money and credit _must be a dynamic theory_. It follows, further, that a theory like the "quantity theory of money," which rests in the notions of "static equilibrium" and "normal adjustment," abstracting from the "transitional process of readjustment," touches the real problems of money and credit not at all. This thesis has seemed to require statistical verification, and the effort has been made to measure the elements in trade, to assign proportions for retail trade and for wholesale trade, to obtain _indicia_ of the extent and variation of speculation in securities, grain, and other things on the organized exchanges, and to indicate something of the extent of less organized speculation running through the whole of business. The ratio of foreign to domestic trade has been studied, for the years, 1890-1916. The effort has also been made to determine the magnitudes of banking transactions, and the relation of banking transactions to the volume of trade. The conclusion has been reached that the overwhelming bulk of banking transactions occur in connection with speculation. The effort has been made to interpret bank clearings, both in New York and in the country outside, with a view to determining quantitatively the major factors that give rise to them. In general, the inductive study would show that modern business and banking centre about the stock market to a much greater degree than most students have recognized. The analysis of banking assets would go to show that the main function of modern bank credit is in the direct or indirect financing of corporate and unincorporated _industry_. "Commercial paper" is no longer the chief banking asset. It is not concluded from this, however, that commerce in the ordinary sense is being robbed by modern tendencies of its proper banking accommodation, or that the banks are engaged in dangerous practices. On the contrary it is maintained that the ability of the banks to aid ordinary commerce is increased by the intimate connection of the banks with the stock market. The thesis is advanced--though with a recognition of the political difficulties involved--that the Federal Reserve Banks should not be forbidden to rediscount loans on stock exchange collateral, if they are to perform their best services for the country. The quantity theory of money is examined in detail, in various formulations, and the conclusion is reached that the quantity theory is utterly invalid.