Tradecraft, or the recruiting and running of agents in the field to gather clandestine information and disrupt the enemy, is rightly associated with secret rendezvous and invisible ink. This intelligent study in British security examines the development of MI5 tradecraft during the interwar period, vital in combatting both Nazi Germany and Soviet Russia, and considers the success of the intelligence service’s most effective sources, including Walter Krivitsky and Maxwell Knight.