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.

Hair came in a great variety of thicknesses, lengths and colors, but usually lost its hue with old age. As it was made of a living material, all hair would grow and fall when the cells died. In younger beings, soon after the cells died, new ones grew again in the same place, making new hair. The regrowth process became less efficient with old age however, eventually leading to baldness.

After regenerating from his ninth body, the Tenth Doctor was pleased to find he had longer hair than his previous incarnation, who wore his hair close-cropped, unlike his other incarnations who had had longer hair, exclaiming "I'm not bald!" (TV: Children in Need Special) Later on, however, he would be dissapointed to be told that his hair wasn't ginger. (TV: The Christmas Invasion) After his next regeneration, the Eleventh Doctor, whose hair was even longer, initally believed that he was "a girl" due to its length, he quickly found otherwise. As it was now longe enough to pull in front of his eyes, he checked the color and to dismay he was "still not ginger." (TV: The End of Time)

Some, such as several incarnations of the Master, chose to grow and style their facial hair.

Strax once mistakenly thought that the hair of a human is actually his face. This implying that Sontarans can't grow hair, although most of them have beards. (TV: The Name of the Doctor)