 The entry is organized around the 1962 popularisations of the concept of incommensurability by Kuhn and Feyerabend and the basic ideas that influenced their developments of this concept. First, Kuhn’s notion of incommensurability as it was initially developed is characterized, as is its cause and its purported consequences. That is followed by an examination of Kuhn’s route to the idea, and then his subsequent development after 1962. The sections on Feyerabend’s notion of incommensurability mirror the same basic structure. They are followed by a brief comparison of Kuhn and Feyerabend’s views on incommensurability, especially its relation to theory comparison.