Skin cancer is the most common type of cancer in the U.S. It occurs in more than a million people each year, including many older people. There are three main types of skin cancer: basal cell carcinoma, squamous cell carcinoma, and melanoma. Of the three, melanoma is the most serious. Skin cancer occurs when abnormal cells form and multiply in an uncontrolled way in the epidermis, or abnormal cells from the epidermis invade the dermis of the skin. Basal cell carcinoma, squamous cell carcinoma, and melanoma are skin cancers that are named for the epidermal cells from which they develop.