This paper describes a qualitative study which explored the views about challenging behaviour of staff in day centres for people with learning difficulties. Respondents' abstract definitions stressed the relativity of the concept. However, they readily cited concrete illustrate incidents of challenging behaviour which involved physical, sexual and verbal assault, noise, non-compliance, non-communication and exposure to danger. Staff explained specific incidents variously, but mono-causally, in terms of service users' dispositions, circumstances and interaction strategies. Respondents' own actions were included in explanatory schema only rarely. Despite its nebulous definition, challenging behaviour became organisationally fixed through staff allocation methods, and through a regulatory system designed to prevent abuse. Staff, caring for large numbers of service users en masse, faced a risk management dilemma. Labelling an individual as challenging could forewarn colleagues about possible dangers. However, such strategies could damage the quality of services which a labelled person received, exacerbating the initial problem.