Background: ACP involving a facilitated conversation with a health or care professional is more effective than document completion alone. In policy, there is an expectation that health and care professionals will provide ACP support, commonly within their existing roles. However, the potential contributions of different professionals are outlined only broadly in policy and guidance. Research on opportunities and barriers for involving different professionals in providing ACP support, and feasible models for doing so, is currently lacking. Methods: We identified twelve healthcare organizations aiming to offer system-wide ACP support in the United States, Canada, Australia and New Zealand. In each, we conducted an average 13 in-depth interviews with senior managers, ACP leads, dedicated ACP facilitators, physicians, nurses, social workers and other clinical and non-clinical staff. Interviews were analyzed thematically using NVivo software. Results: Organizations emphasized leadership for ACP support, including strategic support from senior managers and intensive day-to-day support from ACP leads, to support staff to deliver ACP support within their existing roles. Over-reliance on dedicated facilitators was not considered sustainable or scalable. We found many professionals, from all backgrounds, providing ACP support. However, there remained barriers, particularly for facilitating ACP conversations. A significant barrier for all professionals was lack of time. Physicians sometimes had poor communication skills, misunderstood medico-legal aspects and tended to have conversations of limited scope late in the disease trajectory. However, they could also have concerns about the appropriateness of ACP conversations conducted by others. Social workers had good facilitation skills and understood legal aspects but needed more clinical support than nurses. While ACP support provided alongside and as part of other care was common, ACP conversations in this context could easily get squeezed out or become fragmented. Referrals to other professionals could be insecure. Team-based models involving a physician and a nurse or social worker were considered cost-effective and supportive of good quality care but could require some additional resource. Conclusions: Effective staffing of ACP support is likely to require intensive local leadership, attention to physician concerns while avoiding an entirely physician-led approach, some additional resource and team-based frameworks, including in evolving models of care for chronic illness and end of life. [ABSTRACT FROM AUTHOR]