Within professional higher education, the construct of assessment validity is used to make assumptions about the extent to which students are able to replicate in professional practice what they have learned during their studies through the provision of authentic simulated opportunities to practice. Drawing on the work of Bruno Latour, this article argues that the conceptualisation as well as use of the idea of assessment validity in theorising the assessment of simulation-based learning in professional courses, in order to predict the future performance of the student constitutes a category mistake that consequently makes claims for assessment validity which are unfounded. The article goes on to explore ways by which ethnographers of education might use other elements of Latour's work in order to generate rich, problematising accounts of educational practice.