Human hair growth is regulated by many hormones, and the effect of androgens, in particular, has been well known for a long time. Androgens stimulate beard growth but suppress hair growth in androgenetic alopecia (AGA). This unique condition is known as ‘androgen paradox’. Human hair follicles possess enough enzymes to form the active androgens testosterone and dihydrotestosterone (DHT). 5a-reductase type 1 and 2, androgen receptors (AR) and AR coactivators, in particular, can regulate androgen sensitivity of dermal papillae (DP). To regulate hair growth, androgens stimulate production of IGF-1 as positive mediators and of TGF-b1, TGF-b2, dickkopf1 and IL-6 as negative mediators. In addition, androgens enhance inducible nitric oxide synthase from occipital DP cells and stem cell factor for positive regulation of hair growth in beard and negative regulation of balding DP cells. Moreover, AGA involves crosstalk between androgen and Wnt/b-catenin signalling. Estrogens, another important hormone are shown to inhibit hair growth in animal model, while it extends anagen and shortens catagen in human. This implicates that there are differences in animal and human hair in terms of estrogen responsibility of hair follicle. 17β estradiol, a powerful estrogen, suppresses DHT formation and promotes conversion of testosterone to estradiol by inhibiting 5α -reductase. It also suppresses conversion of androstenedione to testosterone by inhibiting 17β dihydrogenase. On the other hand, it promotes conversion of testosterone to estradiol by stimulating aromatase, so that testosterone production is reduced. As results, DHT production is reduced. It is also known to promote directly differentiation of hair matrix cells. Among constituents of the skin, hair follicle is an organ where nerve fibers with the highest density are distributed. The cyclic growth of the richly innervated hair follicle offers a model for probing such functions in a complex, developmentally regulated tissue interaction system under the physiologic condition. Dissecting the role of neuropeptides in this system may also reveal as yet obscure neural mechanisms of hair growth control. Neuropeptides, which are secreted by nerve fibers, have important roles in the hair growth and hair cycle change. In vitro, substance P (SP) and calcitonin gene related peptide (CGRP) act as growth factor. SP has growth-stimulatory effect and prolongs the anagen phase without affecting the rate of linear hair growth. On the other hand, CGRP has a growth-inhibitory effect, especially in shortening the duration of the anagen phase in hair follicles and DP cells.