The question of how far a state should authorise its agents to go in seeking and using secret intelligence is one of the big unresolved issues of public policy for democracies today. The tension between security and privacy sits at the heart of broader debates concerning the relationship between the citizen and the state. The public needs-and wants-protection from the very serious threats posed by domestic and international terrorism, from serious criminality, to be safe in using cyberspace, and to have active foreign and aid policies to help resolve outstanding international problems. Secret intelligence is widely accepted to be essential to these tasks, and to be a legitimate function of the nation state, yet the historical record is that it also can pose significant ethical risks. Principled Spying lays out a framework for thinking about public policy in this area by clarifying the relationship between ethics and intelligence, both human and technical. In this book, intelligence expert Mark Phythian teams up with the former head of Britain’s GCHQ signals and intelligence agency to try to resolve the knotty question of secret intelligence-and how far it should be allowed to go in a democratic society.