The hair is mainly composed of amino acids, forming from the outside to the inside the hair microdermis, hair cortex, hair fibers, hair myelin …… The chemical composition of hair is mainly protein, water, trace elements, fat, pigment, etc.; hair growth cycle is 2-7 years, regression period is 3 weeks, resting period is 3 months. Thyroid hormone, androgen, growth hormone, etc. have an impact on hair growth 100,000-150000 hair follicles in the whole head, each hair follicle grows 1-3 hairs, 90% of hair follicles are in anagen phase, hair growth is about 25px per month, 40-100 hairs are shed per day; hair growth takes 4-5 months on the top of the head, and slower on the occipital area. There are various factors that affect the quality of hair: perm, dry hair dryer, bleaching and dyeing hair, ultraviolet light, humidity, seawater sweat, air pollution, swimming pool chlorine and other chemicals can affect the quality of hair. Alopecia areata is mainly divided into congenital and acquired, here we mainly talk about acquired alopecia, it is also divided into scarring and non-scarring, scarring is divided into primary and secondary, primary alopecia areata mainly includes discoid lupus erythematosus, follicular lichen planus, pseudo baldness, balding folliculitis, etc.; secondary alopecia areata mainly includes scleroderma, ringworm, scalp trauma, etc. . Non-scarring alopecia include baldness, androgenetic alopecia, resting alopecia, plucking fetish, systemic lupus erythematosus, syphilis, and alopecia areata. Non-scarring alopecia is more common, with baldness, androgenetic alopecia and resting alopecia being clinically more common. The causes of various alopecia vary and treatment varies, so it is best to treat them symptomatically and effectively based on diagnosis.