The Ministry for State Security (German: Ministerium für Staatssicherheit, MfS) or State Security Service (Staatssicherheitsdienst, SSD), commonly known as the Stasi, was the official state security service of the German Democratic Republic (East Germany). 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 (German: 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 CPSU. Erich Mielke was the Stasi's longest-serving chief, in power for thirty-two of the GDR's forty years of existence.