Over the long run, two major forces seem to have been most influential in the past and seem most likely to continue to shape the research agendas of social scientists in the future. The first of these is theory, the second practice. By theory I mean the entire stock of ideas, speculations, conjectures, and recommendations making up the great conversations, or the many little conversations, that proceed within the various disciplines of social science and that give structure and coherence to what social scientists at any time think about what is going on in the world. By practice I mean simply what is going on in the world. The notion that this article seeks to explore in a preliminary way is that when the world changes there is an incentive to change the conversations of social scientists too. New problems come into focus and and new opportunities arise to look again at once-settled issues. Social scientists who seek to understand the workings of the contemporary American political system will have no difficulty in grasping the import of this message. Even a nation that has enjoyed internal stability over the last halfcentury arguably as great or greater than any on earth no revolutions, no coups, no armies of occupation, no constitutional upheavals -has nevertheless experienced profound and continuous change not merely in the superficialities