This article examines the challenges that superdiversity and complexity pose for social workers. Taking an ethnographic approach, we focus on the ‘knowledge-in-action’ of social workers in a small service organization in Belgium in order to access their experiences of being professionals in superdiverse contexts. The reflections of the social workers reveal the prominence of three inter-related issues: the social vulnerability of clients, the tensions that arise in coping with differences between personal and professional frameworks and identities, and the discontinuity that challenges the professional self-confidence of social workers. The findings raise important questions for the professional identity of social work.