Hair

Hair was a part of the anatomy that grew all over the body on many creatures and beings, mostly mammals. Humans and other similar species grew hair in all manner of places except for the soles of their feet and the palms of their hands. Mr Grange had hair growing out of his ears. (PROSE: Time and Relative)

After regenerating from his ninth incarnation, the Tenth Doctor was pleased to find that he had longer hair than his previous incarnation. The Ninth Doctor had worn his hair close-cropped, unlike his other incarnations who had had longer hair. The Tenth Doctor happily exclaimed, "I'm not bald!" (TV: Children in Need Special) Later on, however, he was disappointed to be told that his hair wasn't ginger. (TV: The Christmas Invasion) He decided to try back-combing it for a while, (TV: Fear Her) but later switched to spiking it up. Wilfred Mott used it as an identifying feature of the Doctor while the Silver Cloak was searching for him. He referred to it as "modern sort of hair, all sticky-uppy." (TV: The End of Time)

After his next regeneration, the Eleventh Doctor, whose hair was even longer, initially believed that he was "a girl" due to its length; he quickly determined otherwise. As it was now long enough to pull in front of his eyes, he checked the colour and to his dismay found he was "still not ginger." (TV: The End of Time)

The Eighth Doctor had long locks of hair after regenerating, but cut it down later in his life. (TV: Doctor Who, AUDIO: Tangled Web) It had grown back close to its original length when he regenerated. (TV: The Night of the Doctor)

Melody Pond was excited after regenerating into her third and would-be final incarnation when it came with a very large amount of hair. (TV: Let's Kill Hitler)

Mutation into a Primord from exposure to Stahlman's ooze caused humans to begin growing large amounts of hair on their bodies. A sign the transformation was beginning was excessive hair growth on the hands, and later, the face. (TV: Inferno)

Facial hair
Facial hair was the presence of hair on one's chin, upper lip, or temples.

Some, such as at least two incarnations of the Master, grew and styled their facial hair. (TV: Terror of the Autons, The Mind of Evil, The Keeper of Traken, Logopolis) This beard became a trademark factor associated with the Master, such that the Fifth Doctor asked his future self if the Master still exhibited this trait. (TV: Time Crash)

Some incarnations of the Doctor also ended up with facial hair, but not always out of preference. Sometimes they were aged to the point it caused the unnatural growth of a beard, or they grew one from being kept in captivity with no way of shaving their hair. (TV: The Leisure Hive, Day of the Moon, The Wedding of River Song)

The Fifth Doctor grew a beard while he was stranded in 1867 London to make himself appear older and fit in with the scientific community. (AUDIO: The Haunting of Thomas Brewster)

The Eighth Doctor had facial hair during multiple periods of his life. He grew a beard shortly before his wedding to Scarlette (PROSE: The Adventuress of Henrietta Street) until later shaving it. (PROSE: Hope) He had a consistent state of facial hair while he was stranded on Orbis for six hundred years. (AUDIO: Orbis) In the later years of his life, he started to grow stubble on his face. (TV: The Night of the Doctor)

When he became elderly, the War Doctor grew out a full beard. He maintained this beard up to his regeneration. He was chided with the nickname "Granddad" by the Eleventh Doctor because of his resemblance to a curmudgeonly old man. (TV: The Day of the Doctor)

Sideburns were a variant of facial hair, thick patches of whiskers on the side of one's head. The Tenth Doctor was delighted to see he had grown sideburns after his regeneration. (TV: Children in Need Special) He kept them to the end of his life, (TV: The End of Time) though he was once temporarily aged to a point he lost them, and later, all of his hair. (TV: The Sound of Drums, Last of the Time Lords)

Baldness
Baldness was a condition where an individual's hair on the top of their head began to recede with age, or was absent due to other reasons. It could be acute, such as a small loss of hair, or advance to the point where someone did not have any hair at all. Baldness could also be a hairstyle by choice, afflicted by physical scarring, (TV: The Caves of Androzani) radiation, (TV: The Mutants) or due to biological experimentation that removed hair. (WC: The Making of the Gunslinger, A Town Called Mercy)

When he had grown quite older, the First Doctor appeared to have some degree of baldness, with most of his hair growing around the back of his head. (TV: An Unearthly Child)

The Fifth Doctor gained a temporary bald spot when the time differential in a future version of his TARDIS caused him to age very quickly. (TV: Time Crash)

After the Time Lords resurrected him, the Master's new incarnation was totally bald. The Eighth Doctor ridiculed him because of it. (AUDIO: Eyes of the Master) When the Tenth Doctor was forced to age by the Master, he went partially bald. The second time he was aged, he went completely bald. (TV: The Sound of Drums, Last of the Time Lords)

Strax, a Sontaran without any hair on his own head, didn't understand the growth of hair on humans and other species; he once mistakenly thought River Song's curly hair meant she had a big head. Clara Oswald subsequently referred to River as "the lady with the funny name and the space hair". (TV: The Name of the Doctor)

The Eleventh Doctor once got bored and shaved his head with a razor. However, it allowed him to wear a wig and hide a TARDIS key in his coif that Tasha Lem could not discover when he set foot on a planet with a technology ban. He also exhibited true baldness at the echelon of old age. This condition reversed when his new regeneration cycle reset him back to a youthful body. (TV: The Time of the Doctor)

Greying hair
For some species, such as humans, hair would turn grey, or sometimes entirely white with old age. Various incarnations of the Doctor experienced grey hair. Some, such as the Third Doctor and Twelfth Doctor, had it immediately upon regeneration. (TV: Spearhead From Space, The Time of the Doctor) Others lived to the point their hair went grey, or in the case of the First Doctor, white. (TV: An Unearthly Child, Doctor Who, The Day of the Doctor, The Time of the Doctor, PROSE: Not in My Back Yard) Some were forced to age to the point of grey hair. (TV: The Leisure Hive, The Sound of Drums, Last of the Time Lords)

In rare cases, traumatic events could result in hair turning grey. The Second Doctor's encounter with the Players caused his hair to do such. (PROSE: World Game)

Stuart Hyde's hair went grey when the Master's use of TOMTIT caused a side effect that accelerated his natural aging. Likewise, TOMTIT's continued use reverted him back to his normal age. (TV: The Time Monster)

Jack Harkness, after becoming immortal, still exhibited grey hair as he grew older. (TV: Last of the Time Lords)

Behind the scenes

 * Jon Pertwee adopted a bouffant hairstyle that grew larger with each season he was on Doctor Who. Writer Terrance Dicks claimed this was to hide a prevalent bald spot.