With the arrival of the Thirteenth Doctor coming at the end of this year's Doctor Who Christmas special, it seems a good time to review the mysterious process that will bring him, or her, to viewers. Here's all the must-have facts on regeneration.
1. The Doctor First Regenerated At The End Of 'The Tenth Planet,' The First Story To Feature The Cybermen.
At this point in time, the process was simply referred to as "renewal," implied to be a rejuvenation of cells rather than a full change in body and mind.
2. The Word 'Regeneration' Was Not Used Until the 1974 Story 'Planet Of The Spiders'
This was the final episode for Jon Pertwee, featuring his Third Doctor regenerating into the Fourth, played by Tom Baker.
3. Each Time Lord Or Lady Has A Cycle Of 12 Regenerations, With A Total Of 13 Incarnations
This cycle can be extended at the discretion of the Time Lords in power at the time. The Eleventh Doctor received a new regeneration cycle in "The Time Of The Doctor," while The Master/Missy is said to have received one during the Time War.
4. The Tardis Is An Important Aid To The Doctor During Regeneration
The exact reason why has not been explained. It may have something to do with Gallifreyan technology, or proximity to the Time Vortex.
5. The Behavior Of A Recently Regenerated Individual Can Be Erratic And Unpredictable
The most frightening example of this occurs in "The Twin Dilemma," when the recently regenerated Sixth Doctor attempts to strangle his companion, Peri.
6. To Date, Only Three Doctors Have Regenerated Due To Old Age
They are the First Doctor, The War Doctor and The Eleventh Doctor. The Eleventh Doctor was to have been the final incarnation until a plea from Clara Oswald to the Time Lords prompted them to save him.
7. Only Four Characters Other Than The Doctor Have Regenerated Onscreen
Romana, The Master, River Song and a Time War General. Of these, River is the only one to have done so more than once.
8. There Have Been Hints That Time Ladies Have More Control Over Their Regeneration Than Time Lords
When the first Romana becomes the second, she is seen "trying on" faces before settling on one she likes. Moments before Mels regenerates into River Song, she claims to be focusing on a dress-size.
9. It Is Possible For One's Race To Change During Regeneration
Both River and The General have changed race after regenerating.
10. It Is Also Possible To Change Genders
The once male Master has now returned to the series as the female Missy, having regenerated off screen. The General regenerates from male to female onscreen during "Heaven Sent," with dialogue implying that this was the second gender swap for the character.
Will the #ThirteenthDoctor be female? No one yet knows, but it is a definite possibility.