Mental disorders are overrepresented in the criminal justice system, and this applies equally to police custody. These environments are complex and often pressured, and the acuity of the situation, combined with underlying mental disorders, comorbid medical problems and substance misuse, can lead to behavioural disturbance and increased psychiatric risk. Police custody may also present an opportunity to identify and signpost people with mental disorders and vulnerabilities who are ordinarily hard to reach by standard health services. This article considers the purposes of mental health screening of detainees in police custody. It gives an overview of research into screening for a range of psychiatric disorders and vulnerabilities (including substance misuse and traumatic brain injury) and summarises data on deaths in and immediately following release from custody. Given the inadequacy of statutory screening procedures in some jurisdictions, the authors offer a pragmatic evidence-based protocol to guide screening for mental disorders in custody detainees.