Foucault’s 1970–71 lectures at the Collège de France, The Will to Know, highlight the significance of themes of purity and impurity in Western thought. Reflecting on these themes coincided with the emergence of Foucault’s theory of power. This article presents the first analysis of Foucault’s investigation of purity and impurity in The Will to Know lectures, identifying the distinctive theory Foucault offers of purity as a discursive apparatus addressing correspondence between the subject and the truth through the image of relative integrity or mixture. It then traces Foucault’s subsequent reflections on these themes in his later writings on disciplinary power. The implications of Foucault’s position are considered; the article will close by putting Foucault’s ideas in dialogue with those of Kristeva, and in considering the role that purity and impurity may play in resistance.