Abstract: Despite many attempts to evaluate the effectiveness of case management for frail older people, systematic reviews including experimental designs show inconsistent results. Starting from the view that case management is a complex intervention o c- cur ring in multilayered realities, we conducted a realist evaluation of case manag e- ment in Belgium, where this type of intervention is new. Realist approaches are pa r- ticularly well suited to evaluate complex interventions as they seek to investigate it e- rative ly the literature and empirical data to uncover mid -range theories underpi n- ning the intervention under study. As such, realist evaluations are works in progress which provide tools to describe how, why and for whom an intervention is supposed to work. In t his paper, we describe two mid - range theories that can explain why case management can help frail older people to remain at home, through the lens of c a- pacity and social support.