Baldness (synonym: Alopecia) is the state of lacking hair where it usually would grow, especially on the head.

The most common form of baldness seen is male pattern baldness, in which the hair recedes from the lateral sides of the forehead, known as "receding brow". Additionally, a bald patch can develop on top (vertex). The trigger for this type of baldness, which is also known as androgenetic alopecia, is currently believed to be an enzyme, 5-alpha reductase, that converts the hormone testosterone into dihydrotestosterone (DHT), which inhibits hair growth. The age at which the enzyme appears, if it does at all, is genetically determined. Male pattern baldness is being classified on the Hamilton-Norwood scale I-VIII.

Female pattern baldness, in which the midline parting of the hair appears broadened, is less common. It is believed to result from a decrease in estrogen, a hormone that normally counteracts the balding effect of testosterone, which normally occurs in women's blood. Female pattern baldness is being classified on the Ludwig scale I-III.

Hair loss is a frequent side-effect of chemotherapy, in all ages and both sexes. This is due to the fact the chemotherapy affects all rapidly replicating cells, including the ones in the hair follicle.

Some mycotic infections can cause massive hair loss.

Other forms of baldness:

Alopecia areata
John D. Rockefeller had an extreme case of alopecia that caused him to lose all of the hair on his face, including his eyebrows and eyelashes. Another famous person who suffers from similarly severe alopecia is Italian football referee Pierluigi Collina.