The Ministry for State Security (German: Ministerium für Staatssicherheit, MfS, German: [ɛmɛfˈʔɛs] (listen)), or State Security Service (Staatssicherheitsdienst, SSD), commonly known as the Stasi (German: [ˈʃtaːziː] (listen)), was the official state security service of the German Democratic Republic (East Germany, GDR). It has been described as one of the most effective and repressive intelligence and secret police agencies to have ever existed.The Stasi was headquartered in East Berlin, with an extensive complex in Berlin-Lichtenberg and several smaller facilities throughout the city. The Stasi motto was Schild und Schwert der Partei (Shield and Sword of the Party), referring to the ruling Socialist Unity Party of Germany (Sozialistische Einheitspartei Deutschlands, SED) and also echoing a theme of the KGB, the Soviet counterpart and close partner, with respect to its own ruling party, the Communist Party of the Soviet Union (CPSU). Erich Mielke was the Stasi's longest-serving chief, in power for 32 of the 40 years of the GDR's existence.

One of the Stasi's main tasks was spying on the population, primarily through a vast network of citizens-turned-informants, and fighting any opposition by overt and covert measures, including hidden psychological destruction of dissidents (Zersetzung, literally meaning "decomposition"). It arrested 250,000 people as political prisoners during its existence. Its Main Directorate for Reconnaissance (Hauptverwaltung Aufklärung) was responsible both for espionage and for conducting covert operations in foreign countries. Under its long-time head Markus Wolf, this directorate gained a reputation as one of the most effective intelligence agencies of the Cold War. The Stasi also maintained contacts, and occasionally cooperated, with Western terrorists.Numerous Stasi officials were prosecuted for their crimes after 1990. After German reunification, the surveillance files that the Stasi had maintained on millions of East Germans were opened, so that all citizens could inspect their personal file on request. The files were maintained by the Stasi Records Agency until June 2021, when they became part of the German Federal Archives.