Although many patients with critical illness may benefit from involvement of palliative care specialists, adoption of these services in the intensive care unit (ICU) is variable. To characterize reasons for variable buy-in for specialty palliative care in the ICU, and identify factors associated with routine involvement of specialists in appropriate cases. Qualitative study using in-depth, semi-structured interviews with ICU attendings, nurses, and palliative care clinicians, purposively sampled from eight ICUs (medical, surgical, cardiothoracic, neurological) with variable use of palliative care services within two urban, academic medical centers. Interviews were transcribed and coded using an iterative and inductive approach with constant comparison. We identified three types of specialty palliative care adoption in ICUs, representing different phases of buy-in. The “nascent” phase was characterized by the need for education about palliative care services and clarification of which patients may be appropriate for involvement. During the key “transitional” phase, use of specialists depended on development of “comfort and trust”, which centered on four aspects of the ICU-palliative care clinician relationship: 1) increasing familiarity between clinicians; 2) navigating shared responsibility with primary clinicians; 3) having a collaborative approach to care; and 4) having successful experiences. In the “mature” phase, ICU and palliative care clinicians worked to strengthen their existing collaboration, but further adoption was limited by the availability and resources of the palliative care team. This conceptual framework identifying distinct phases of adoption may assist institutions aiming to foster sustained adoption of specialty palliative care in an ICU setting.