This study evaluates the oversight of national security and intelligence agencies by parliaments and specialized non-parliamentary oversight bodies, with a view to identifying good practices that can inform the European Parliament's approach to strengthening the oversight of Europol, Eurojust, Frontex and, to a lesser extent, Sitcen. The study puts forward a series of detailed recommendations (including in the field of access to classified information) that are formulated on the basis of in depth assessments of: (1) the current functions and powers of these four bodies; (2) existing arrangements for the oversight of these bodies by the European Parliament, the Joint Supervisory Bodies and national parliaments; and (3) the legal and institutional frameworks for parliamentary and specialized oversight of security and intelligence agencies in EU Member States and other major democracies.