Present-Day economic historians should be well qualified to talk about uncertainty. There was a happier time when the empirical and concrete study of individual situations was deemed sufficient, when mediodological problems of the long run were left for historians to raise and solve if they could and controversies centered on questions that they themselves formulated. Differences there were, but few irreconcilable differences; areas of the unknown were recognized but there appeared to be no gaps in knowledge that could not be bridged in the course of time. Now, for better or worse, this sheltered position has itself become a datum of history. The challenge of social scientists seeking chics to an understanding of die forces shaping secular change has become increasingly insistent and effective; the strenuous wooing of Clio by economic theorists, statisticians, and Parsonian sociologists can no longer be ignored. Assuming that retreat to the shelter of cloistered storytelling is out of the question, creative response on the part of historians to the uncertainties of their twentieth-century environment becomes a condition of leadership in historical inquiry, its lack a virtual guarantee of submergence.