Chameleon Arch

The Chameleon Arch was Time Lord technology which could modify the biology of an individual, essentially changing their species. Connected to a TARDIS' databank via a cable, the arch resembled a headset incorporating a biodata module usually resembling a fob watch. The module was placed on the front on the Time Lord's forehead. The watch 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, provided by a TARDIS. To ensure that the biodata module 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 both with or without modifying their outer appearance. (TV: Human Nature / The Family of Blood; AUDIO: The Heavenly Paradigm)

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. (TV: Human Nature) heard drums beating in his head all his life, a burden the Master also faced. (TV: Utopia, The Sound of Drums, The End of Time)

Words like "TARDIS" and "regeneration" and other words of Time Lord lore stirred up residual memories in Professor Yana, (TV: Utopia) while John Smith kept a dream journal in which he chronicled memories of the Doctor's adventures. (TV: Human Nature / The Family of Blood)

Reading a text message from Lee Clayton triggered an instinct from her true identity in Ruth Clayton, causing her to recall the lighthouse where she grew up and to defend herself unarmed when cornered by Judoon. Getting closer to her biodata module, a fire alarm, she could hear whispers of "break the glass." (TV: Fugitive of the Judoon)

Though he had become a human, Quarren Maguire was still time sensitive, and unlike his wife, he sensed, if only briefly, when the timelines changed as the Last Great Time War caught up with them. (AUDIO: One Life)

The Eighth Doctor claimed that destroying a Chameleon Arch while the user was not a Time Lord would render their Time Lord personality and biology irretrievable. (AUDIO: One Life) The Time Lord essence stored in a Chameleon Arch could observe and communicate thoughts to their human self, (TV: Utopia, AUDIO: The Heavenly Paradigm) or to others with psychic abilities, (TV: Human Nature) through direct physical contact. (TV: Human Nature, Utopia)

While Quarren indicated that he could not return to his human personality, (AUDIO: One Life) the Tenth Doctor later told Joan Redfern that he could change back, but actively chose not to. (TV: The Family of Blood)

Notable uses
An extremely early incarnation of the Doctor, prior to his First known incarnation, once used a chameleon arch to escape a service to Gallifrey that "one did not apply for". Using the arch, the Doctor hid out on Earth and took on the human identity of "Ruth Clayton". She had her TARDIS buried near the lighthouse where she stored her true identity, and trusted Lee Clayton to protect her, under cover as Ruth's husband. Lee was also tasked with reminding her to "follow the light and break the glass" when the time was right, as this would lead Ruth to return to the lighthouse, which she was made to recall from childhood, and finally restore her Time Lord memories. While being chased by the Judoon and Gat and following an encounter with the Thirteenth Doctor, Ruth eventually followed the instructions and became the Doctor once again. (TV: Fugitive of the Judoon)

There was an account suggesting that 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 Time Lord who would become Quarren Maguire was a powerful psychic with the ability to manipulate reality, but he used the Chameleon Arch to escape the Time War as he didn't want to fight. Quarren hid his true Time Lord self in a locket that he shared with his wife, Rupa Maguire, and created a history for the pair of them. After the Daleks attempted to track him down, his identity was exposed by Ollistra while he was attempting to escape with various other human refugees in the Doctor's TARDIS. Determined to save the Eighth Doctor, Quarren opened the Arch and restored himself to his original state, rewriting history to save the TARDIS and send most of the human refugees to safety and the other Time Lords to Gallifrey. Quarren then erased himself from history, as he could not go back to the good man he was. (AUDIO: One Life)

used the Arch to turn into a human infant to escape the Last Great Time War, after witnessing the Daleks take over the Cruciform. (AUDIO: The Heavenly Paradigm) Calling himself Professor Yana, he remained in human form for long enough to grow into adulthood and even old age, until he opened the biodata module and remembered who he was. Due to the effects of the module's perception filter, Yana did not notice anything unusual about his fob watch until it was pointed out by Martha Jones. After Martha pointed out the watch to Yana and he heard familiar phrases from Martha, the Tenth Doctor and Captain Jack Harkness, Yana opened the watch and resumed his identity as the Master. (TV: Utopia)

used a Chameleon Arch to flee the Time War, living as a human in a 16th century English abbey with his TARDIS's components disassembled. (AUDIO: Divorced, Beheaded, Regenerated)

The Ninth Doctor gave a biodata module to Plex. (COMIC: The Promise)

The Tenth Doctor used a Chameleon Arch to change himself into the human John Smith to elude the Family of Blood. He left Martha Jones in charge of protecting John Smith, and instructed her to use the fob watch biodata module in case of danger. (TV: Human Nature) Due to the threat of the Family of Blood, John reluctantly opened the watch and resumed his identity as the Doctor. (TV: The Family of Blood)

The Tenth Doctor used a representation of 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
During the Last Great Time War, the extremely psychic Time Lord who would adopt the name Quarren Maguire used a Chameleon Arch to make himself human, fearing his abilities would be used for the destruction of more than just the Daleks. His biodata module took the form of a pendant. (AUDIO: One Life)

Narvin gave a Chameleon Arch for providing the Time Lords with Swenyo. (AUDIO: Sins of the Father)

Later in the Time War, Romana II and Narvin stole a Chameleon Arch from a TARDIS in the wreckage of Dreadnought Septima. (AUDIO: Hostiles) To prevent the Daleks from using her to track her fleeing friends, Romana considered using this Arch on Unity to become an ordinary farmer. (AUDIO: Unity)

Following the disappearance of Gallifrey, (TV: The Day of the Doctor) there were some Chameleon Arches remaining in the universe:
 * One belonged to the Doctor inside his TARDIS, though he gave the biodata module fob watch to Timothy Latimer. (TV: Human Nature / The Family of Blood) In light of its later appearance in the Eleventh Doctor's drawing room, (GAME: TARDIS) he had more than one, as the Ninth Doctor gave a modified one to Plex that broke down by the time the Twelfth Doctor retrieved it. (COMIC: The Promise)
 * Another was 's, which he used to escape the Time War, hiding out at the end of the universe. (TV: Utopia)
 * A third was 's, who also fled the Time War in this way. (AUDIO: Divorced, Beheaded, Regenerated)

Behind the scenes

 * Interestingly, a fob watch strongly resembling a biodata module is one of the objects seen in the Doctor's Gladstone bag in the 1996 Doctor Who TV movie, as he retrieves his sonic screwdriver to seal the Master's remains.
 * Also in the TV movie, among the Doctor's belongings examined by Chang Lee was a normal fob watch.