I have interpreted the rather facetious title given me by the programme committee as permission to speak rather light-heartedly, even irresponsibly, about a few of the changes in the formal methods of analysis in the field of economic theory; to speak of fashions in tools, fashions in gadgets, fashions, if you will forgive me, in models.In the 1920's Professor J. M. Clark supported the thesis that economic theory has developed by replacing worn and outgrown half-truths by new half-truths, that correspond more closely with increased knowledge, changing points of view, and different circumstances. Sir Dennis Robertson reviewed the first volume of the Survey of Contemporary Economics, called his review “A Revolutionist's Handbook,” and enumerated seven revolutions; but his phrasing is ironical. In his posthumous work on the History of Economic Analysis Professor Schumpeter has contended, with some success I think, that advances in economic analysis have been more consistent and less fluctuating than changes in the field of economic doctrine, economic systems, or economic thought. Fashions change, then, even in the methods of economic theory, but there is a good deal of continuity too. Many of the most widely useful tools of economic theory have changed very little in the last generation, or even in the last century; some have been in use for many centuries. Nevertheless, there have been many important changes too. Periodically, as attention comes to be focussed on a particular sort of problem, old tools are repaired, polished up, and put in the shop window; or new ones are invented.