Mental health is now acknowledged as the ‘core business of policing’ (Adebowale, 2013, p. °6), with a third of people in police custody estimated to have some form of mental health problem (Leese and Russell, 2017). While the Police and Crime Act 2017 places restrictions on the use of police cells as ‘places of safety’, it is important to remember that the police must still process, as well as care for, those they arrest who have mental health problems. The detention of those with suspected mental health problems is governed by PACE 1984, but research suggests that there are many problems with identifying and managing detainees with mental disorder in custody (Noga et al., 2016). The number of people in custody who are under the influence of alcohol or drugs have added to these challenges, as has limited police training (Cummins, 2016). Moreover, the risks involved with detaining vulnerable people in police custody have been subject to increasing high-profile scrutiny (Adebowale, 2013; Independent Advisory Panel on Deaths in Custody, 2015). In this chapter our primary focus is with how arrestees with possible mental health problems are processed in police custody, the protections available to them and the key challenges involved with their detention. The chapter explores the relevant provisions under PACE 1984 along with consideration of key actors including the custody sergeant, healthcare professional and appropriate adult. Through an examination of these roles we explore the challenges and risks that surround the detention of suspects with mental disorder in police custody.