The Doctor is a mysterious, time-traveling alien hero from the planet Gallifrey who belongs to a species called Time Lords

Played by: William Hartnell, Patrick Troughton, Jon Pertwee, Tom Baker, Peter Davison, Colin Baker, Sylvester McCoy, Paul McGann, Christopher Eccleston, David Tennant, Matt Smith, Peter Capaldi, Jodie Whitakker and Ncuti Gatwa.

Starring: William Hartnell, Patrick Troughton and Jon Pertwee.

Doctor Who celebrated its 10th Anniversary with a special episode titled "The Three Doctors" - it was the very first time on the show that any former Doctor Who had returned and would become the standard for any future anniversary episode.

Starring: William Hartnell (Archeive Footage), Richard Hurndall, Patrick Troughton, Jon Pertwee, Tom Baker (Archive Footage) and Peter Davison.

Doctor Who celebrated its 25th Anniversary with the special episode titled "The Five Doctors" - it followed the standard for an anniversary episode with former lead stars returning.

The First Doctor had to be recast due to William Hartnell's passing and Tom Baker's Fourth Doctor only appeared in archieve footage.

Starring: David Tennant, Matt Smith, John Hurt and Peter Capaldi. 

(All Classic Doctors appear in Archive Footage)

Doctor Who's next anniversary celebration would be its 50th titled "The Day of the Doctor" - the episode would see the Tenth and Eleventh Doctor's meet a previously unknown incarnation that existed during The Time War.

Starring: David Tennant, Ncuti Gatwa and Catherine Tate

The 60th Doctor Who year of Doctor Who would see the return of David Tennant, Catherine Tate and Showrunner Russell T Davies.

Unlike Previous Anniversary Years, the 60th would contain three specials starring Tennant and Tate with no other former lead actors returning.

The Final Episode of the specials would introduce Ncuti Gatwa as the new lead actor.