This paper explores the significance of the concept of power/knowledge in educational theory. The argument proceeds in two main parts. In the first, I consider aspects of Stephen J. Ball's highly influential work in educational theory. I examine his reception of Foucault's concept of power/knowledge and suggest that there are problems in his adoption of Foucault's thought. These problems arise from the way that he settles interpretations into received ideas. Foucault's thought, I try to show, is not to be seen in a confined way. In the second part, I seek a different reading of Foucault's notion of power/knowledge in order to break with this tendency to confine, referring to the work of Gilles Deleuze. I draw particularly on Deleuze's thought of the outside as a means of manifesting the significance of power/knowledge in relation to processes of subjectification. At the end of the paper, I suggest how educational theory might be reconceived in the light of potencies of power/knowledge that the paper has demonstrated.