This paper presents a new theoretical model which conceptualizes inter-professional and multi-agency collaborative working, at the level of the individual within a group. This arises from a review of the literature around joint working, and is based on social psychological theories which refer to shared goals. The model assumes that collective commitment, collective efficacy, and process and outcome beliefs interact and feed into the development and maintenance of collaborative processes and outcomes. This is situated within an ecological framework that summarizes the context of inter-professional and multi-agency collaboration. The model illustrates working resolutions of specific inter-professional dilemmas around identity, role and control. This paper extends the literature around theoretical approaches to collaborative work in a multi-agency or inter-professional context, with its specific social-psychological focus on the motivations of the individual within the group.