The T-54 and T-55 series (variants of the same design) have the distinction of being the most widely produced tanks in history, exceeding even the wartime production runs of the T-34 and M4 Sherman when Soviet as well as license-production is included. The original T-54 was designed as a successor to the T-34 medium tank but boasted an unprecedented combination of armament, armor, and speed which arguably made it one of the first main battle tanks even if it was not initially considered as such. The T-54 was the first Soviet tank which adopted the characteristic dome-shaped cast turret that provided excellent ballistic protection for its time despite its cramped nature compared to taller but roomier Western tanks. The relatively small profile was also an advantage although this limited the gun's depression, a problem also seen in its successors. Armor along the frontal arc was exceptional for a medium tank of its time, all the more considering this was achieved with a weight of less than 40 tons which was significantly less than its main NATO contemporaries like the British Centurion and US Patton series. Armament of the T-54/55 was based around the 100-mm D-10T rifled gun, which had been previously seen in an earlier incarnation in the wartime SU-100 assault gun. This was competitive with contemporary NATO guns, at least until the introduction of the British L7 in the late 1950s, but hull limitations meant that it could not be upgraded with higher caliber guns later in its production run. A 12.7-mm DShK heavy anti-aircraft machine gun was fitted on most variants (oddly removed on the T-55 initially) while the T-54 and original T-55 variant were also the last Soviet tanks to feature a bow machine gun. A novel defensive feature was the ability to lay a smoke screen by injecting diesel fuel into the exhaust, a feature that remained present in all subsequent diesel-fueled Soviet tanks. Though absent on the T-54, the T-55 became the first Soviet tank with NBC protection to provide survivability in a nuclear environment. More advanced fire control systems including laser designators as well as appliqué and explosive reactive armor and even early active protection systems were also added to keep the Soviet Army's large inventories of these vehicles competitive into the 1980s. The end of the Cold War, however, saw most of them relegated to storage or sold off. Though it entered Red Army service shortly after the end of World War II, mass production of the T-54 did not begin until the end of the 1940s with the T-55 following up a decade later. License production of the T-54 took place in China (as the Type 59), and of the T-55 in Poland and Czechoslovakia. Up to 50 countries eventually adopted it for service, and it saw action in the Indo-Pakistani and Arab-Israeli wars (where it proved to be roughly equal to the US M48), the Vietnam War, the Iran-Iraq War, as well as the Gulf War, by which time they were thoroughly outclassed by modern Western designs and as a consequence suffered heavy losses. In Soviet service, T-54/55s were used in large numbers to repress anti-Soviet uprisings in Hungary (1956) and Czechoslovakia (1968), with a number of them destroyed by partisans. T-55s were later deployed during the Soviet-Afghan War in the 1980s as newer tanks were seen as unnecessary for an infantry-based conflict. In Russian service, small numbers have also been reactivated for use in the Russian-Ukraine war, although primarily for fire support while Ukraine has also utilized the Slovenian-upgraded M-55S. Despite its obsolescence, the T-55 remains in service with dozens of countries worldwide thanks to its low cost and good reliability, and availability of upgrades (including Western equipment) in order to remain viable.

The prototype of a successor to the wartime T-44 was completed in 1946 (as Ob'yekt 137), with production commencing the following year. The initial T-54 obr. 1946 featured a more traditional turret, reminiscent of the T-34 and T-44, but limited production gave way to improved versions that corrected some of the design's early problems. The T-54 obr. 1949 was the first true production version and the first to feature the domed turred but with an overhang which was removed in the T-54 obr. 1951. A major upgrade in the mid-1950s resulted in the T-54A with the vertically-stabilized D-10TG gun with power-operated elevation and a snorkel for amphibious operations, and the T-54B with, with a two-axis stabilized D-10T2S as well as night-fighting equipment. By the late 1950s, production switched to the improved T-55 (Ob'yekt 155). This introduced NBC protection as well as a rotating turret floor, while increasing fuel capacity as well as ammunition load. Notably the DShK anti-aircraft gun was removed. The NBC system was further refined in the T-55A which also removed the bow machine gun. The anti-aircraft gun was re-added in the T-55A obr. 1970 while a laser rangefinder was added in the obr. 1981. Other modern versions include the T-55AM with added BDD appliqué armor and the T-55AD with the Drozd anti-tank missile system. Converted T-54s to T-55 standard were known as the T-54M. A further modernization program took place in the 1980s and resulted in the T-55M with a new fire control system, engine, suspension and equipment as well as the ability to carry 9K116-1 (AT-10) anti-tank missiles. Some later received Kontakt-1 ERA and were designated T-55AV and T-55MV, while the T-55AMV included both ERA as well as BDD armor. The T-55M5 and T-55M6 were developed after the dissolution of the USSR and incorporated new equipment and engines as well as the more advanced Kontakt-5, the latter with a lengthened chassis. There are further upgraded available for export, and which incorporate some of the features of more modern tanks like the T-80. Command versions (of which there were numerous sub-variants) were known as the T-54K and T-55K series. Other variants include the OT-54 and TO-55 flame-thrower tanks, the BTS series of armored recovery vehicles, the SPK-12G heavy crane, the MTU-1 and MTU-20 bridgelayers, and the IMR combat engineer vehicle. An assault gun known as the SU-122 was fitted with a 122-mm gun but most were later converted into ARVs, additionally the BTR-T was a heavy APC based on the T-55's chassis. Foreign variants are too numerous to list but most operators eventually incorporated local improvements and modifications (the Chinese Type 59 and Type 69 are listed separately). Captured Israeli tanks were known as the Tiran series, some with domestically-produced guns.