The shaft of a hair consists of a cuticle and a cortex of hard-keratin surrounding a soft-keratin medulla. Pigmented hair contains melanin in the cortex and medulla. The colour of hair depends mainly on the shade and the amount of pigment in the cortex and on air spaces in the hair. In white hairs, the pigment is absent from the cortex, and the contained air is responsible for the whiteness.

The root of a hair is situated in an epidermal tube known as the hair follicle, sunken into either the dermis or the subcutaneous tissue. The follicle is dilated at its base to form the bulb (matrix). Between the root of a hair and the surface of the skin, a bundle of smooth muscle fibers, known as an erector pili muscle extends from the deep part of the hair follicle to the papillary layer of the dermis. On contraction, it makes the hair erect. The arrectores pilorum are innervated by sympathetic fibers and contract in response to emotion or cold.

The functions of hair include protection, regulation of body temperature, and facilitation of evaporation of perspiration; hairs also act as sense organs. The hairs first developed constitute the lanugo, which is shed shortly before birth. The fine hairs that develop later constitute the vellus. In a few places (such as the palms and the soles and the dorsal aspect of the distal phalanges) the skin is glabrous.