The Type 205 was a class of German diesel-electric submarines. They were single-hull vessels optimized for the use in the shallow Baltic Sea. The Type 205 is a direct evolution of the Type 201 class with lengthened hull, new machinery and sensors. The biggest difference though is that ST-52 steel is used for the pressure hull since the Type 201's non-magnetic steel proved to be problematic. Type 206, the follow-on class, finally succeeded with non-magnetic steel hulls.

The Type 206 is a class of diesel-electric submarines (U-boats) developed by Howaldtswerke-Deutsche Werft (HDW). Its design is based on the preceding Type 205 submarine class. These small and agile submarines were built during the Cold War to operate in the shallow Baltic Sea and attack Warsaw Pact shipping in the event of military confrontation. The pressure hulls were built out of non-magnetic steel to counter the threat of magnetic naval mines and make detection with MAD sensors more difficult. The low emission profile allowed the submarines in exercises to intrude even into well-protected opposing forces such as carrier formations with their screen.