Comorbidity with behavioral health conditions is highly prevalent among those experiencing serious medical illnesses and is associated with poor outcomes. Siloed provision of behavioral and physical healthcare has contributed to a workforce ill-equipped to address the often complex needs of these clinical populations. Trained specialist behavioral health providers are scarce and there are gaps in core behavioral health competencies among serious illness care providers. Core competency frameworks to close behavioral health training gaps in primary care exist, but these have not extended to some of the distinct skills and roles required in serious illness care settings. This paper seeks to address this issue by describing a common framework of training competencies across the full spectrum of clinical responsibility and behavioral health expertise for those working at the interface of behavioral health and serious illness care. The authors used a mixed-method approach to develop a model of behavioral health and serious illness care and to delineate seven core skill domains necessary for practitioners working at this interface. Existing opportunities for scaling-up the workforce as well as priority policy recommendation to address barriers to implementation are discussed.