Hair is a filamentous outgrowth of dead cells from the skin, found only in mammals. Although many other life forms, especially insects, show filamentous outgrowths, these are not considered "hair" regarding the accepted meaning of the term.

The most noticeable part of human hair is the hair on the head, which can grow longer than on most mammals and is more dense than most hair found elsewhere on the body. The average human head has about 100,000 hair follicles. Its absence is termed alopecia, commonly known as baldness or hair loss.

Hair care for humans is a major world industry with specialized tools, chemicals and techniques. The business of various products connected with human hair has become an important industrial and financial factor in Western societies.

It is easier to prevent the falling out of healthy hairs than to regrow hair in follicles that are already dormant. Finasteride and minoxidil have shown some success in partially reversing loss. In a one-year study of finasteride, evaluation after one year showed five of 21 subjects had two-grade improvement in MNHS grade on a modified Norwood/Hamilton scale and 12 of 21 subjects had one-grade improvement; the others remained at the same grade.

However such treatments are generally ineffective at treating extreme cases of hair loss.