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In a few short hours we say goodbye to the 10th Doctor played by David Tennant.  While the actor might be saying goodbye, the role will be passed on to Matt Smith.  Back in 1966 to continue Doctor Who from the ailing William Hartnell, the story writers wrote about changing one's appearance when the Doctor's body was damaged beyond normal healing.  They used the term "renewal" for this process until the 3rd Doctor, played by Jon Pertwee, transformed into the 4th Doctor, played by Tom Baker.  At that time they called it "regeneration."  

IO9 has a great write up on all the regenerations of the Doctor, including videos of the change.