Few television programmes have achieved the same level of cultural significance as Doctor Who, one of the most extensive and well-known works of science fiction.
As the British series approaches its 60th anniversary, it has seen significant changes since its 1963 debut.
The First Doctor, played by 55-year-old William Hartnell, established the bar for what later iterations would build upon.
Possibly even more significant than the First Doctor's debut was Patrick Troughton's as the Second. The Doctor's ability to assume a new form, regeneration, was also mentioned in his introduction.
The series underwent a number of daring casting decisions in the 1970s that shocked viewers. The Third Doctor, played by Jon Pertwee, and the Fourth Doctor, played by Tom Baker, were both a few years younger than the previous two Doctors.
K9, who was voiced by John Leeson, rose to become one of the most well-known and enduring friends of all time.
Compared to the last group of Doctors, his casting provided a stronger boost. He had lost the dedication to the Doctor's more formal attire and appeared young and bold.
With Russel T. Davies in charge, Doctor Who eventually made its way back to television in 2005 after a decade of little to no episodes.
By surpassing Davison as the youngest Doctor by taking on the role at barely 27 years old, Matt Smith reached the pinnacle of the show's youthfulness in both production and fandom.
When Jodie Whittaker assumed the position of the Thirteenth Doctor, she became the first female Doctor.