 Was there a Scientific Revolution that replaced pre-scientific thinking about nature and society and thus marked the transition to modernity? Which later developments, if any, are truly revolutionary? Are attributions of revolution usually a sign of insufficient historiographical understanding? In any case, how are such episodes to be explained historically and epistemologically? Are they contingent, that is, historical accidents and thus perhaps avoidable; or are they somehow necessary to a “progressive” science? And, if so, why? Is there an overall pattern of scientific development? If so, is it basically one of creative displacement, as Kuhn claimed? Do all revolutions have the same structure and function, or are there diverse forms of rupture, discontinuity, or rapid change in science? Do they represent great leaps forward or, on the contrary, does their existence undercut the claim that science progresses? Does the existence of revolutions in mature sciences support a postmodern or “post-critical” (Polanyi) rather than a modern, neo-Enlightenment conception of science in relation to other human enterprises? Does their existence support a strongly constructionist versus a realist conception of scientific knowledge claims? Are revolutions an exercise in rationality or are they so excessive as to be labeled irrational? Do they invite epistemological relativism? What are the implications of revolution for science policy? This entry will survey some but not all of these issues.