Anagen
During the anagen stage of the hair growth cycle the cells at the follicle base divide and form the new hair which is then pushed up to the surface of the skin. Once the hair has been completely formed the cells at the base of the follicle stop dividing and the hair stops growing.

Catogen
The hair then moves into the catogen stage where it is still nourished and remains firmly attached.

Telogen
Finally the hair moves into the telogen stage where the follicle no longer nourishes the hair and therefore the hair is shed. The follicle remains dormant for a certain period of time before the cycle is repeated.

The hair growth cycle A = Anagen C = Catogen T = Telogen (courtesy of www.afraidtoask.com)

Anagen hair	Catogen hair	Telogen hair

Depending on where the hair follicle is on the body the length of each of these growth stages will vary. The follicles on the scalp remain in the anagen stage the longest time and therefore the hair on the head grows the longest.

The laser light can only destroy hair when it is in the anagen phase and therefore has visible pigment. When the hair is in the catogen or telogen phases it is not visible and capable of absorbing the laser light and therefore cannot be destroyed.

This alongside the fact that hair follicle growth is not synchronised and therefore different hairs follicles will be in different hair growth stages explains why laser hair removal requires multiple treatments.