The T-54/55 series eventually became the most-produced tank in history. Estimated production numbers for the series range from 86,000 to 100,000. The T-54/55 series was eventually replaced by the T-62, T-72, and T-80 in the Soviet and Russian Armies, but tanks of the series are still in use by up to 50 other armies worldwide, some having received sophisticated retrofitting. T-54 and T-55 tanks never directly faced their NATO Cold War adversaries in Europe, however, their first appearance in the west in 1960 spurred the United States to develop the M60.