Nurses working in police custody settings may be called on to assess a detainee’s fitness to be interviewed by police, for example where the person is thought to have a mental disorder or vulnerability. This article outlines the role of the custody nurse in the fitness-to-interview assessment. This assessment is complex and multifaceted. It requires custody nurses to assess a detainee’s medical, mental health, educational and social history, as well as any substance misuse. It should include a mental state examination and, where appropriate, a physical examination. Fitness-to-interview decisions should be rigorous to prevent miscarriages of justice or significant harm to detainees. Custody nurses should advocate for detainees who are mentally vulnerable and ensure appropriate safeguards are in place, where necessary. Further work is required to reduce the subjectivity of the fitness-to-interview assessment, along with increased investment in appropriate adult services. Equally, custody nurses working at this advanced level of practice require relevant postgraduate knowledge and skills.