This chapter reflects on the relationship between national security, surveillance, and human rights. The possibility to adopt restrictive measures in case of threats to national security is an essential component of the international legal system and one of those ‘adjustment variables’ that allow international human rights law to accommodate and ascertain its social functions. The abuse of these mechanisms, however, is also a major threat to the legal order, judicial security, and the rule of law. Domestic rulers and security agencies have often used ‘national security’ as a pretext to violate human rights and fundamental freedoms; to monitor political opponents; to conceal embarrassing or illegal behaviour; to bypass investigation by independent and democratic bodies; or to suppress political and social unrest. In a democracy, surveillance must be balanced with public liberties, and especially with privacy. The chapter looks at the case law of the European Court of Human Rights on national surveillance. It presents the three main criteria used by the Court to assess the compatibility of surveillance laws with the European Convention of Human Rights.