Hair Loss or baldness, includes a set of disorders which involve the state of lacking hair where it would normally grow, especially on the head. The most common form of baldness is a progressive hair thinning condition called androgenic alopecia or 'male pattern baldness' that occurs in adult human males and some primate species. The severity and nature of baldness can vary greatly; it ranges from male and female pattern alopecia (androgenetic alopecia, also called androgenic alopecia or alopecia androgenetica), alopecia areata, which involves the loss of some of the hair from the head, and alopecia totalis, which involves the loss of all head hair, to the most extreme form, alopecia universalis, which involves the loss of all hair from the head and the body. Treatment for alopecia has limited success. The more hair lost, the less successful the treatment will be.

Male pattern baldness is thought to occur in varying forms in about 66% of adult males at some point in their lives.  It is characterized by hair receding from the lateral sides of the forehead, known as "receding hairline" or "receding brow." An additional bald patch may develop on top (vertex). The trigger for this type of baldness (called androgenic alopecia because it is caused by male hormones or androgens) is DHT, a powerful sex hormone. The mechanism by which DHT accomplishes this is not yet understood. In genetically-prone scalps, DHT initiates a process of follicular miniaturization. Through the process of follicular miniaturization, hair shaft width is progressively decreased until scalp hair resembles fragile vellus hair or "peach fuzz" or else becomes non-existent. Onset of hair loss sometimes begins as early as end of puberty, and is mostly genetically determined. Male pattern baldness is 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 classified on the Ludwig scale I-III.