 The difficulties in identifying and conceptualizing scientific revolutions involve many of the most challenging issues in epistemology, methodology, ontology, philosophy of language, and even value theory. With revolution we immediately confront the problem of deep, possibly noncumulative, conceptual and practical change, now in modern science itself, a locus that Enlightenment thinkers would have found surprising. And since revolution is typically driven by new results, or by a conceptual-cum-social reorganization of old ones, often highly unexpected, we also confront the hard problem of understanding creative innovation. Third, major revolutions supposedly change the normative landscape of research by altering the goals and methodological standards of the enterprise, so we face also the difficult problem of relating descriptive claims to normative claims and practices, and changes in the former to changes in the latter.