Social work is an institution that promises effects: its service users should be better off after a measure than before. The paper addresses the related assumption of causality. The central causal mechanism of social work, it is argued, lies in the interactions between professionals and service users. This kind of causality can do justice to the complexity of social work and the agency of service users. In a critical examination of evidence-based approaches, the contribution argues for the development of a concept of causality that is sustainable for social work in order to explain why it has an impact. Two special features of social work are important in this respect: it is confronted with relatively high complexity and service users shape the effects of social work in their interactions with professionals. A social work conceptualisation of causality should consider both aspects, and it should be analytically grounded in interactions between professionals and service users. However, their interrelated practices do not stand alone. Interactions are always reflexively accompanied by the participants. The awareness and expectation that social work measures (should) have consequences is part of the practice of social work and its effects.