When suspects are arrested, they spend their time in police custody largely in isolation and out of public view. These custody blocks are police territory, and public controversies about what happens there often only arise when a detainee dies. Custody visitors are volunteers who make what are supposed to be random and unannounced visits to police custody blocks to check on the welfare of detainees. However, there is a fundamental power imbalance between the police and these visitors, which calls the independence and effectiveness of custody visiting into question. Investigating this largely unexplored part of the criminal justice system, this timely book includes the voices of the detainees who have a unique insight into the scheme. It offers detailed proposals for radically reforming custody visiting to make it an effective regulator of police behaviour, with an explanation of the political context that could make that a reality.