Chameleon Arch

The Chameleon Arch was Time Lord technology which could modify the biology of an individual, essentially changing that individual's species. Connected to a TARDIS databanks via a cable, the arch resembled a headset incorporating a Fob watch. The watch was placed on the front on the Time Lord's forehead. The watch then stored the Time Lord's essence and when opened, would return it to the Time Lord. The arch gave the Time Lord a new personality and memories. To ensure that the watch would not be noticed, it was incorporated with a perception filter. Using the arch re-wrote a Time Lord's cellular biology and was therefore painful. The Tenth Doctor screamed in agony when undergoing transformation into a human. It was capable of altering them without modifying their outer appearance. (TV: Human Nature / The Family of Blood)

Function
Some traits of the user's true Time Lord self still remained after the use of the Chameleon Arch. For example, John Smith was able to use a simple cricket ball to deliberately cause a complex chain of events to save a mother and her baby from being crushed by a falling piano. Smith also kept a dream journal in which he chronicled memories of the Doctor's adventures. (TV: Human Nature/The Family of Blood) heard drums beating in his head all his life, a burden the Master also faced. (TV: Utopia/The Sound of Drums/Last of the Time Lords)

Notable uses
There was an account suggesting the Seventh Doctor, prior to his regeneration into his eighth incarnation, made use of a broken Arch to create the fiction that he was half-human to use against the Master. (COMIC: The Forgotten, TV: Doctor Who)

The Tenth Doctor used a Chameleon Arch to change himself into the human John Smith to elude the Family of Blood. (TV: Human Nature/The Family of Blood) The Master also used the Arch to turn into a human to escape the Last Great Time War. He remained in human form until he opened the watch and remembered who he was. (TV: Utopia)

The Tenth Doctor used his Chameleon Arch to recover his memories, after his mind was taken over by Es'Cartrss of the Tactire. (COMIC: The Forgotten)

The Tenth Doctor also briefly considered that Jackson Lake might be a future incarnation of himself under the influence of an Arch. Lake had what turned out to be just an ordinary fob watch. The initials on the back of the fob watch revealed his true identity to both himself and to the Doctor. (TV: The Next Doctor)

Known Chameleon Arches
After the destruction of Gallifrey, there were only two known Chameleon Arches in the universe. One belonged to the Doctor inside his TARDIS, though he gave the fobwatch to Timothy Latimer. (TV: Human Nature/The Family of Blood) He seemed to have either recovered it or had duplicates, in light of its later appearance in the Eleventh Doctor's drawing room. (GAME: TARDIS) The other was in the possession of before he left the end of the universe to the 21st century. (TV: Human Nature, Utopia)

Behind the scenes

 * Interestingly, when the Doctor collects his sonic screwdriver from a bag to seal the box containing the Master's remains in the Doctor Who TV movie, a Chameleon Arch is one of the items seen at the top of the bag.
 * Also in the TV movie, among the Doctor's belongings examined by Chang Lee, was a normal fob watch.