This chapter examines how the Combined Services Detailed Interrogation Centre (CSDIC) in Britain developed its sophisticated system of interrogating German soldiers. While CSDIC’s use of microphones is now widely known, the process that produced its intelligence remain poorly understood. The chapter shows that CSDIC was initially designed to gather pieces of information overheard among soldiers, but eavesdropping alone soon proved insufficient for the scale and complexity of the war. Soon enough, CSDIC UK had become the final step in a much larger screening process that sifted hundreds of thousands of soldiers to identify a select group for the bugged sites. The actual process in its camps then formed a cycle: statements obtained in direct interrogation were checked against private conversations between prisoners, which in turn revealed new leads for direct interrogation. Microphones, therefore, were part of a broader interrogation method rather than its entirety. The chapter also highlights the people at the centre of this system, whose work yielded a remarkable body of intelligence, yet its emotional and psychological toll remained largely concealed by secrecy well after the war.