The 501st Legion was one of the oldest Stormtrooper legions active within the Empire's Stormtrooper Corps. Originally being made up purely of Clones, the 501st gradually gained more and more standard human recruits within its ranks as opposed to clones. Eventually, the 501st gained a reputation for being one of the best Stormtrooper legions within the Empire, gaining the nickname, Vader's Fist.
They fought for the Galactic Republic, and later fighting for the early Galactic Empire as some of the first stormtroopers before the Legion was decomissioned with the rest of the Clone Army, just before the start of the Galactic Civil War. During its time with the Republic and Empire, the Legion was led by Clone Captain Rex, Darth Vader, and Commander Appo.