Intelligence officers – generally known as ‘spies’ – will not openly admit, even within their own family circles, that they work for an intelligence service. These agencies have neither local offices nor facilities, nor do they visit schools to talk about the daily life and work of their officers. Public media appearances are extremely rare, and it is usually only the director of the agency whose face is seen in public. Innate secretiveness and the inaccessibility of many official archives are obstacles to transparent research on intelligence services. In 1998, I commenced my doctoral thesis on the Spanish intelligence services. Far from being a passing adventure, the topic became my principal line of research for over two decades, during which time I completed almost 250 interviews, including 100 with members and ex-members of the intelligence services and another 50 with heads of government, Spanish and foreign intelligence service directors, military staff, politicians, and diplomats. In this chapter, I present my fieldwork experience of interviewing intelligence officers and the evolution of a young and somewhat naïve researcher into a more mature and distrustful one.