© 2018, © 2019 Institute of Materials, Minerals and Mining Published by Taylor & Francis on behalf of the Institute. This article reflects on issues arising from attempts to treat behaviour as an object of scientific and social scientific study. It examines what happens when behaviour is taken as a thing, an object of concern, modification and enquiry. At the heart of the notion of behaviour, this article argues, lies a fundamental ambiguity. The concept’s power, but also its elusiveness, lies in its ability to tack back and forth between two visions: on the one hand behaviour as materialized, objectified action, regular, repetitive and rule-bound, and on the other behaviour as a placeholder, a word to index something we do not yet know or understand. Those are two ways of being a ‘thing’.