The incommensurability thesis played a central role in Kuhn's philosophy of science. In 1962, he introduced it in the Structure of Scientific Revolutions to make sense of nonsensical statements in antiquated scientific texts. The philosophy of science community was critical of the thesis, arguing it made science both irrational and relativistic. In response, Kuhn proposed several versions of the incommensurability thesis to clarify and defend it. He also shifted from a historical to an evolutionary philosophy of science; and, with the shift came a change in both the notion and role for incommensurability. Kuhn defined it now with respect to changes in the lexical taxonomy of a scientific specialty. He also ascribed to it the function of isolating one scientific specialty's lexicon from another's, and he used it to underpin a notion of scientific progress as the proliferation of scientific specialties. In this chapter, I reconstruct Kuhn's evolving notion of incommensurability and its role, and critically analyze how he employed its mature version to address scientific truth and reality.