The Doctor is an alien who can "regenerate" from fatal injuries into a new body. This fact that has allowed for the show's exceptional longevity. Doctor Who was launched in 1963, and has continued with different leads and despite lengthy breaks right up to the present day.

Matt Smith has been named as the Eleventh Doctor, and took over from the Tenth Doctor, played by David Tennant, at the conclusion of a special two-part story called The End of Time. The first part aired in Britain on Christmas Day, 2009, with part two following on on New Year's Day.http://www.bbc.co.uk/doctorwho/s4/features/bulletins/bulletin_091203_01

The next series of Doctor Who will be shown on the BBC in the spring of 2010.

1. William Hartnell
2. Patrick Troughton
3. John Pertwee
4. Tom Baker
5. Peter Davison
6. Colin Baker
7. Sylvester McCoy
8. Paul McGann
9. Christopher Eccleston
10. David Tennant
11. Matt Smith

The internal look of the TARDIS has changed several times during the series history. Typically we see only a small portion of the interior, mainly its "Console Room" where the main controls and front door are locared. However the series implies that the TARDIS is vast in size, and we occasionally see other parts of it, such as enormous wardrobes where the Doctor finds suitable clothing for himself and his companions when dressing to blend in with the local culture is required..

The TARDIS console room suffered damage during the Tenth Doctor's regeneration into his Eleventh incarnation, providing the grounds for a re-designed interior which will feature in the next series of Doctor Who, starting in Spring 2010.

The most important companions to feature since the reboot include Rose Tyler, Jack Harkness, Martha Jones and Donna Noble.

A former companion from the classic series, Sarah Jane Smith, made an appearance in the epsiode School Reunion, and was subsequently given a spin-off series of her own, aimed especially at younger children.

The Daleks are a race of genetically engineered organic beings who each live within the shell of a personal travel vehicle, which is both heavily armed and armoured. They are super-intelligent, extremely technologically advanced, capable of time travel themselves, and utterly ruthelss. Their mission is to eradicate all who are not like them, whom they see as inferior vermin. Their famous battle cry is "EXTERMINATE!"

The Cybermen are a cybernetic/human fusion. They are humans whose brains have been implamented within a steel robotic body, and modified to remove emotions such as fear. The Cybermen often seek to "upgrade" other humans in similar fashion, but those that they deem unsuitable for conversion are instead simply "deleted". Hence their battle cry is often: "Delete! Delete! Delete!"

The Master is a Time Lord like the Doctor, and the two were childhood friends. However the Master developed psychotic tendencies and became bent on the domination of others, rejoicing in power and destruction. Consequently he has often been one of the Doctor's most cunning and dangerous enemies. Nevertheless, their former friendship is not entirely forgotten. The Doctor is concerned to try to redeem the Master if he can, and the Master often seems to have mixed feelings of affection and rage towards the Doctor.

Like the Doctor, The Master has gone through several regenerations during the course of the series.