Moles are brown or black growths, usually round or oval, that can appear anywhere on the skin. They can be rough or smooth, flat or raised, single or in multiples. They occur when cells that are responsible for skin pigmentation, known as melanocytes, grow in clusters instead of being spread out across the skin. Generally, moles are less than one-quarter inch in size. Most moles appear by the age of 20, although some moles may appear later in life. Most adults have between 10 and 40 moles. Because they last about 50 years, moles may disappear by themselves over time.