Background: Advance care planning (ACP) is the process of discussing and documenting wishes and preferences for future care. Research about ACP for people with intellectual disabilities (ID) is limited. This study describes what is important for ACP in the palliative phase of people with intellectual disabilities. Method: In-depth interviews were conducted with people with intellectual disabilities (n = 5), relatives (n = 7) and professional caregivers (n = 8). Qualitative data were analysed inductively, using the principles of thematic analysis. Results: Important themes in ACP were as follows: tailoring care, working as a team and taking and giving time. The perceived role of people with intellectual disabilities in ACP was to express their wishes. Relatives had a signalling, representing and contributing role. Professionals felt their role was to inform, collaborate and coordinate. Conclusions: A staff training programme about ACP should cover how to build and maintain close relationships, provide a safe environment and address ACP as an integral part of care.