This article argues that security politics is constituted through the ways in which its contours are ‘made sensible’ (perceivable to our senses) through material, aesthetic, or affective mechanisms. To make this argument, the article introduces the theoretical scaffolding for what it terms a compositional ontology. A compositional approach to security identifies, theorises, and studies the perceptual base of security politics in order to ask how – say – the sight of a single photograph, sound of a security announcement, or smell of tear gas, can frequently be the direct (efficient) cause of international security policy, discourse, and decision. To theorise the deep political impact of such fleeting moments of local sensory experience, the article lays out a compositional ontology comprised of a synthesis of poststructuralist and new-realist philosophy, as well as the empirical sensibilities of pragmatist sociology. Combined with a focus on sensibility, it is shown that these approaches can produce an ontology of security that more effectively explains contingency, fluidity, and change in world politics. Having laid out the theoretical frame for a compositional ontology, the article discusses its political and methodological implications, suggesting it demands that security studies shift towards a more postcritical, experimental, and collaborative ethos.