There has been a renewed interest in collaborative models of health care delivered by ‘interdisciplinary teams’ of providers across several health care systems. This growing phenomenon raises a host of issues related to the management of professional boundaries and the contemporary state of medical dominance. In this paper, we undertake a critical analysis of the factors both promoting and impeding collaborative care models of primary and mental health care in Canada and the USA. The data our arguments are based upon include a combination of documentary and interview data from key stakeholders influential in various collaborative care initiatives. Based on these data, we develop a conceptual model of the various levels of influence, focusing in particular on the macro (regulatory/funding) and meso (institutional) factors. Our comparative policy and institutional analysis reveals the similarities and differences in the influences of the broader contexts in Canada and the USA, and by extension the different ways that the structural embeddedness of medical dominance impinges upon and reacts to recent policy changes regarding collaborative health care teams.