In recent decades in general and since the turn of the millennium in particular, accountability has emerged as the democratic state’s most important legitimation mechanism for national security and intelligence practices and institutions. The functional imperative of accountability is to scrutinize intelligence agencies, practices, and systems to ensure their compliance with regulations, mandates, laws, and the Constitution—largely to provide reassurance that organisations that must operate in secret are not abusing their special powers and authorities. In addition to compliance, however, many other benefits that flow from crucial subsidiary functions that intelligence accountability performs. This chapter tracks the value and importance of intelligence to detect, disrupt, and deter a growing number of threat actors and vectors to security, prosperity, and democracy, and concomitant shift in intelligence posture from foreign spies to an array of domestic counterterrorism, counterespionage, counter sabotage, counter propaganda, counter information and counter cyber operations. In an era of social and political disruption that is marked by technological change, changing threat vectors, changing public expectations and scandals, democratic governments have established, evolved, and expanded accountability systems to review and oversee the intelligence community. The chapter introduces readers to ways of theorizing and reconciling the relationship between the intelligence community and civilian institutions of the democratic state, notably as a principal–agent problem: a relationship in which one side (the civilians) attempts to have an epistemic community (the ‘spies’) carry out its will. Politicians and the civil service leverage the intelligence community’s expert knowledge to keep the democratic state, its institutions, and its citizens safe and secure and deter and contain existential threats. In fulfilling this mandate, the democratic public expects the intelligence community to comply with relevant legal and constitutional frameworks.