The Project 633 class (known in the West by its NATO reporting name as the Romeo class) is a class of Soviet diesel-electric submarine, built in the 1950s. At the end of World War II, the Soviets obtained several Type XXIs, from which they were able to obtain certain key technologies. These technologies assisted in the design of the Zulu- and Whiskey-class. Further improvements on the design led to the Romeo class, which was designed by Lazurit Central Design Bureau ("Lazurit" is the Russian word for lazurite).
Only 20 of the Soviet Union's originally intended 56 were completed between October 1957 and the end of December 1961 because of the introduction of the nuclear submarine into the Soviet Navy.
By today's standards Romeo class submarines are considered obsolete, but they still have some value as training and surveillance vessels.