Chameleon Arch

The Chameleon Arch was a piece of Time Lord equipment which could modify the biology of a creature so the cells were that of something else. The procedure was very painful. It was composed of a headset and a fob watch-like device, with an added perception filter, used to store the Gallifreyan biological information.

The Doctor used it to change himself into the Human John Smith to escape the Family of Blood. (DW: Human Nature)

The Master also used it to turn into a Human to escape the Last Great Time War, and remained in Human form until he opened the watch part and remembered everything that the Doctor had done to him. (DW: Utopia)