While normal skin cells grow, develop, and die in predictable cycles, skin cancer develops when skin cells grow out of control. Instead of dying, the damaged DNA within skin cancer cells causes them to continue growing and produce more abnormal cells. They also tend to invade other tissues. Skin cancer is the most common type of cancer in the world. Ultraviolet (UV) light exposure, most commonly from sunlight, is overwhelmingly the most frequent cause of skin cancer. Other important causes of skin cancer include the following:

Use of tanning booths
Immunosuppression
Exposure to unusually high levels of X-rays
Contact with certain chemicals-arsenic (miners, sheep shearers, and farmers)
The following people are at the greatest risk:

People with fair skin, especially types that freckle, sunburn easily, or become painful in the sun
People with light (blond or red) hair and blue or green eyes
Those with certain genetic disorders that deplete skin pigment such as albinism, xeroderma pigmentosum
People who have already been treated for skin cancer
People with numerous moles, unusual moles, or large moles that were present at birth
People with close family members who have developed skin cancer
People who had at least one severe sunburn early in life