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The Doctor is a Time Lord, an extraterrestrial from the planet Gallifrey, who wanders time and space in an internally vast time machine called the TARDIS — Time And Relative Dimension(s) In Space. Although the TARDIS once had the ability to disguise itself according to its environment, after landing in 1963 London its facade became "stuck" in the form of a British police box because of a malfunctioning chameleon circuit. It has remained in that shape ever since. Over the course of the series the Doctor occasionally attempts to fix the circuit, most notably in Logopolis and Attack of the Cybermen (in the latter turning the TARDIS exterior into a pipe organ, among other incongruous shapes), but eventually gives up the effort out of fondness for the police box shape. The discrepancy between the small exterior of the ship and its vast interior is explained by its dimensionally transcendental nature, whereby the ship's interior and exterior dimensions exist independently of each other.[1]

Little is known about the Doctor's childhood. In "The Empty Child" the Doctor claimed he knows "what it is like to be the only child left out in the cold". Later, during "The Girl in the Fireplace", Madame de Pompadour "saw" memories of his childhood during a telepathic session between the two and commented that it was "so lonely". However, when asked if he has a brother in "Smith and Jones", the Doctor simply replied "not any more". In the same episode, he mentioned "playing with Röntgen blocks in the nursery." In "The Sound of Drums" (2007), the Doctor describes a Time Lord Academy initiation ceremony where, at the age of eight, Time Lord children are made to look into the Untempered Schism, a gap in space and time where they can view the time vortex. Some are inspired, some go mad (as he suggests happened to his nemesis the Master), and some run away. When asked what he did, he replies, "Oh, one of the ones that ran away - I've never stopped!"

References to the Doctor's family are rare in the series. During the first two seasons he travelled with his granddaughter, Susan Foreman, and as noted above he apparently once had a brother. During his second incarnation when asked about his family, the Doctor says his memories of them are still alive (The Tomb of the Cybermen) but whether that means they are deceased is unknown.

Mostly due to the age and unreliability of the TARDIS's navigation system, the Doctor explores the universe at random; using his extensive knowledge of science and technology to avert whatever crises he encounters. The Doctor generally travels with one or more companions. Most of these make a conscious decision to travel with him, while others, especially early in the series, are accidental passengers.

Although Time Lords resemble humans, their physiology differs in some key respects. For example, like other members of his race, the Doctor has two hearts (binary vascular system), a "respiratory bypass system" that allows him to go without air for some while, an internal body temperature of 15–16 degrees Celsius (60 degrees Fahrenheit)[citation needed] and he occasionally exhibits a super-human level of stamina. Additionally, he has shown a resistance to temporal effects and has demonstrated a telepathic ability, albeit to a limited degree. The Doctor also exhibits some weaknesses uncommon to humans. For example, in The Mind of Evil (1971) he claimed that a tablet of aspirin could kill him.

In his final serial, the Second Doctor states that Time Lords can live forever, "barring accidents." When "accidents" do occur, Time Lords can usually regenerate into new bodies, resulting in extremely long life-spans.

Quite apart from his name, why the Doctor uses the title of "the Doctor" has never been explained on screen. The Doctor, at first, said that he was not a medical doctor, often referring to himself as a scientist or an engineer. However he does occasionally show medical knowledge and has stated that he studied under Joseph Lister and Joseph Bell on separate occasions. In The Moonbase, the Second Doctor mentions that he studied for a medical degree in Glasgow during the 19th Century. He has also been mocked by his fellow Time Lords for adhering to such a "lowly" title as "Doctor", although in The Armageddon Factor he tells Drax that he achieved his doctorate, indicating it was at least a somewhat respectable title. In "The Girl in the Fireplace", he draws an analogy between the title and Madame de Pompadour's. In "The Sound Of Drums", the Master remarks to the Doctor that they both chose their names, and that it was sanctimonious of the Doctor to identify himself as "the man who makes people better".

The Telos novella Frayed by Tara Samms (which takes place prior to "An Unearthly Child") has the First Doctor being given that title by the staff of a besieged human medical facility on the planet Iwa, suggesting at the end that the Doctor liked the official title so much that he adopted it. However, this does not quite explain why the Time Lords use the same title in addressing him. The same story also has Jill, a young girl living in the facility, naming the Doctor's granddaughter "Susan" after Jill's mother. The canonicity of all non-television sources is uncertain.

To make up for his lack of a practical name, the Doctor often relies upon convenient pseudonyms. In The Gunfighters, the First Doctor uses the alias Dr. Caligari. In The Highlanders the Second Doctor assumes the name of "Doctor von Wer" (a German approximation of "Doctor Who"), and signs himself as "Dr. W" in The Underwater Menace. In The Wheel in Space, his companion Jamie McCrimmon, reading the name off some medical equipment, tells the crew of the Wheel that the Doctor's name is "John Smith". The Doctor subsequently adopts this alias several times over the course of the series, often prefixing the title "Doctor" to it. The Eighth Doctor's companion Grace briefly refers to him by the alias "Dr Bowman" in the 1996 Doctor Who television movie.

In "New Earth", it is implied that the Doctor is part of the prophecy of the Face of Boe and is referred to as "The Lonely God". In "Tooth and Claw", having landed in Scotland, the Tenth Doctor introduces himself as "Dr James McCrimmon" from the township of Balamory. James McCrimmon is in fact the full name of the Second Doctor's companion known as Jamie. Later in that episode, the Doctor is knighted by Queen Victoria as "Sir Doctor of TARDIS."

To his greatest enemies, the Daleks, the Doctor is known as the Ka Faraq Gatri, the "Bringer of Darkness" or "Destroyer of Worlds". This is first mentioned in the novelisation of Remembrance of the Daleks by Ben Aaronovitch and subsequently taken up in the spin-off media, particularly the Virgin New Adventures books and the Doctor Who Magazine comic strip. In "The Parting of the Ways", the Doctor claims that the Daleks call him "The Oncoming Storm" — this name is used by the Draconians (whose word for it is "Karshtakavaar") to refer to the Doctor in the Virgin New Adventures novel Love and War by Paul Cornell.

The series has also occasionally toyed with the Doctor's identity (or lack thereof). In the first part of The Mysterious Planet, the Doctor suggests writing a thesis on "Ancient Life on Ravolox, by Doctor...", but is interrupted by Peri. In The Armageddon Factor, the Time Lord Drax addresses the Fourth Doctor as "Thete", short for "Theta Sigma"; later, in The Happiness Patrol, this was clarified as a nickname from the Doctor's University days. In Remembrance of the Daleks the Seventh Doctor produces a calling card with a series of pseudo-Greek letters inscribed on it (as well as a stylised question mark). This may be a reference to Terrance Dicks' and Malcolm Hulke's book The Making of Doctor Who (1972), which claims that the Doctor's true name is a string of Greek and mathematical symbols.

The question mark motif was common throughout the eighties, in part as a branding attempt. Beginning with season eighteen, the Fourth through Seventh Doctors all sported costumes with a question mark motif (usually on the lapels, except in the Seventh Doctor's case on his pullover and the shape of his umbrella handle). In the 1988 serial Remembrance of the Daleks, the Seventh Doctor is asked to sign a document; although the signature itself is not directly seen on screen, his hand movements clearly indicate that he signs it with a question mark.

It was mentioned by Tom Baker, the Fourth Doctor, during an interview with The Age in 2003, that the Doctor is called so because he is "a doctor of time and relative dimension in space".[4] Apart from being called a doctor of the TARDIS, the Doctor has also been referred to as just a "doctor of time travel".[5]