The relationship between the Ministry of State Security and the Polish Ministry of the Interior or its secret service was one of the most difficult within the Warsaw Pact. The volume describes the coexistence and opposition of the two secret services on the basis of extensive source research and a detailed consideration of their diverging interests. Behind the facade of an officially prescribed and contractually agreed friendship, there was abysmal mistrust. The Polish perspective also reveals surprising weaknesses of the Stasi. The time focus is on the years 1974 to 1990, but the study also looks at the early years since 1949. In the frosty relationship of the allied secret services, the problematic relationship between the two 'brother states' is always reflected.