In this timely and clear account, Christopher Nathan explores these questions and more. He rejects the view that the consequences of undercover policing always justify the means, instead advancing an argument that through their actions people can make themselves morally liable to some forms of undercover policing. Drawing on several recent, high-profile case studies, Christopher Nathan argues for a new understanding of proportionality in undercover policing that takes account of innocent parties, vulnerable targets, and manipulation into wrongful action. He also defends a central role for the judiciary in the oversight of undercover policing.