Anthropologists and social theorists have written extensively about the ways in which economic and political interests lead people to act in social life. Less scholarly attention has been paid, however, to what people find ‘interesting’, in the sense of being engaged or enthused, rather than of having a stake in something. Even more rarely have we attended to the ethnographic deployment of both of these ambiguous senses of interest in specific social contexts. This paper explores the intersection of these two versions of interest, highlighting how they are central to a range of expert knowledge practices and to the claims of professionals to know with authority. Focusing on animal rights activists, heritage bureaucrats, and behavioural scientists, we show how these different forms of interest coalesce, combine, and pull apart in practice. We argue that these observations about vernacular deployments of interest should lead us to rethink the use of interest in anthropological analysis. In particular we caution against the unwitting universalization of normative assumptions about the proper relation between interest as enthusiasm and interest as advantage. We highlight instead the need to be alert to the dynamic ways in which these senses of interest are threaded through social practice.