Hair cycling is the rhythmic change of the hair follicle through phases of growth (anagen), regression (catagen), and rest (telogen). Synchronized hair follicle cycling (in mammals) prepares the hair coat for seasonal changes in habitat conditions as well as procreational activities. The purpose of hair cycling in mammals with individual (asynchronous) follicle waves (eg, humans) is not as obvious, but may include cleaning the skin surface of debris and parasites, and excretion of deleterious chemicals by encapsulation within trichocytes. In addition, follicle cycling might serve as a regulator of paracrine or even endocrine secretion of hormones and growth modulators produced within the follicle and secreted into the skin or circulation. Finally, hair follicle cycling may act as a safe-guarding system against malignant degeneration by protecting rapidly dividing keratinocytes from oxidative damage by deletion during catagen. The majority of the known hair growth disorders are a consequence of changes in the hair cycle. The most frequent growth disorder in men and women is androgenetic alopecia(AGA). AGA is characterized by a shortening of the anagen phase and a prolongation of telogen, combined with miniaturization of hair follicles. Hair cycle disturbances have dramatic effects on visible hair growth. If anagen gets prematurely terminated and catagen occurs too early, this must result in effluvium and alopecia; the affected skin region will subsequently sport largely catagen and/or telogen follicles, whose loosely anchored club hairs are eventually shed (i.e. the normal anagen/telogen rate on the scalp[roughly 4:1]changes in favor of telogen). This is exactly what happens, for example, as a consequence of drug-induced damage to the proliferating cells of the anagen hair bulb, such as in drug-induced telogen effluvium or when inflammatory cells attack the anagen hair bulb in alopecia areata. Therefore, the therapeutic manipulation of hair follicle cycling is a key challenge in hair loss management.