Acne is most common in teenagers, but it can occur at all ages. The condition usually begins at puberty and may continue for many years. Three out of four teenagers have acne to some extent, probably caused by hormonal changes that stimulate the sebaceous (oil producing) skin glands.

Other hormonal changes related to menstrual periods, pregnancy, birth control pills, or stress can also aggravate acne.

Acne is caused when glands within the pores of the skin become plugged, because oil and skin cells accumulate faster than they can exit. The plug causes the hair follicle to bulge (causing whiteheads), and the top of the plug may appear dark (causing blackheads). If the plug causes the wall of the follicle to rupture, the oil, dead skin cells, and bacteria found normally on the surface of the skin can enter the skin and form small infected areas called pustules (also known as pimples or "zits"). If these infected areas are deep in the skin, they may enlarge to form firm, painful cysts.

Acne commonly appears on the face and shoulders, but may extend to the trunk, arms and legs.

Acne is not caused by dirt or by masturbation or other activities, but dirt and oil on the face can aggravate the condition. Other factors that increase the chances of acne are hormonal changes, exposure to weather extremes, stress, oily skin, endocrine disorders, certain tumors, and the use of certain drugs (such as cortisone, testosterone, estrogen, and others).

Acne is not contagious. It tends to run in families. A tendency to have acne may persist through ages 30's to early 40's.