The Tango class was the NATO reporting name of a class of diesel-electric submarines that were built in the Soviet Union to replace the Foxtrot-class submarines assigned to the Black Sea and Northern Fleets. The Soviet designation of this class was Project 641B and it was also known as the Som (Catfish) class. The first of the class was completed in 1972 at Gorky. A total of 18 were built in two slightly different versions. The later type was several metres longer than the first, possibly because of the installation of ASW missile equipment.

The Foxtrot class was the NATO reporting name of a class of diesel-electric patrol submarines that were built in the Soviet Union. The Soviet designation of this class was Project 641. The Foxtrot class was designed to replace the earlier Zulu class, which suffered from structural weaknesses and harmonic vibration problems that limited its operational depth and submerged speed. The first Foxtrot keel was laid down in 1957 and commissioned in 1958 and the last was completed in 1983. A total of 58 were built for the Soviet Navy at the Sudomekh division of the Admiralty Shipyard, Saint Petersburg. Additional hulls were built for other countries.