An understanding of policy implementation issues in many of the core fields of social policy requires attention to the mode of service delivery that is characteristic of those fields: face-to-face interaction between frontline service staff and service users. A focus on implementation as interaction between these groups reveals the significance of 'agency' to the analysis of why and how certain social policy outcomes occur -and in particular why outcomes may sometimes be different from those anticipated by formal policy objectives. This article considers how the agency of workers and users shapes the processes of mediation, negotiation and adaptation that occur in different service delivery contexts, and identifies ways in which resistance to intended policy outcomes, and to the practices expected to deliver them, is formed and expressed. Drawing on a wide range of empirical research material, the article shows how agencies of resistance produce outcomes that are uncertain, fragile and contested, and whose effect can be the subversion of formal policy intentions.