Background: Police custody sergeants have a duty to secure an AA to safeguard the rights and welfare of vulnerable people detained or questioned by the police. This study focuses on the role of the AA in supporting vulnerable adults and seeks to examine what stakeholders would expect from an effective AA service. Methods: This was a qualitative study of four AA services in England. Interviews were undertaken with 25 professionals, and two focus groups were held with service-user groups (13 participants). Results: There is disparity between the expectations of professionals, and service users, on what comprises an effective service. Professionals tend to prioritise the availability and response time of AAs, while service users prioritise their personal attributes and demeanour. Conclusions: Professionals involved in commissioning and management of AA services should monitor whether the rights of vulnerable adults in custody are protected and better engage vulnerable adults beyond service delivery.