jats:titleAbstract</jats:title>jats:pThe idea of measurement (of bodies and identities) is a guiding principle of globalized surveillance in the War on Terror. Nevertheless, this inherently scientific notion is so naturalized in public and academic discourse that its meaning and implications are left undiscussed. This paper builds on quantum theory to present an immanent critique of measurement in surveillance. Foregrounding surveillance's transdisciplinary conceptual foundations, it argues that a notion of identity measurement centered on ambiguity and embodiment, rather than fixity and abstraction, reshapes the scope for political action and opens new avenues for critique. I suggest that a lack of critical engagement with the concept of measurement accounts for Surveillance Studies’ and International Political Sociology's difficulty in exploring the relation between the material–affective dimension of surveillance and its identity-fixing function. Challenging unquestioned notions of measurement in social science, quantum theory highlights the interconnected importance of ambiguity and embodiment in processes of identity measurement. Through the case of airport security, I illustrate how quantum measurement departs from recent critical accounts of surveillance—concerned with the fixity of unambiguous identities and the digital abstraction of bodies—and foregrounds the ambiguity of affect to postulate new forms of agency and resistance to the politics of surveillance.</jats:p>