Hair loss or baldness (technically known as alopecia) is a loss of hair from the head or body. Baldness can refer to general hair loss or androgenic alopecia (male pattern baldness). Some types of baldness can be caused by alopecia areata, an autoimmune disorder. The extreme forms of alopecia areata are alopecia totalis, which involves the loss of all head hair, and alopecia universalis, which involves the loss of all hair from the head and the body.

Baldness and hypotrichosis can have many causes, including fungal infection (tinea capitis), traumatic damage, such as by compulsive pulling (trichotillomania), as a result of radiotherapy or chemotherapy, and as a result of nutritional deficiencies such as iron, and as a result of autoimmune phenomena, including alopecia areata and hair loss associated with systemic lupus erythematosus.