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. The Body's Largest Organ The skin is the body's largest organ. It has two main layers: the inner layer, called the dermis, and the outer layer, called the epidermis. The dermis contains sweat glands, nerves, hair follicles, and blood vessels. The epidermis forms the protective, waterproof layer of the skin. The very top of the epidermis, which is called the stratum corneum, is made up of dead cells that have moved their way up through the other layers. The epidermis, or outer layer, is made up of three types of living cells: - Squamous cells are flat and form the top layer of living cells.  - Basal cells are round and lie directly under squamous cells.  - Melanocytes are specialized skin cells that produce pigment called melanin. Squamous cells are flat and form the top layer of living cells. Basal cells are round and lie directly under squamous cells. Melanocytes are specialized skin cells that produce pigment called melanin. The melanin pigment produced by melanocytes gives skin its color. It also protects the skin from ultraviolet (UV) ray damage from the sun by absorbing and scattering the energy. People with more melanin have darker skin and better protection from UV light. People with lighter skin (less melanin) are more vulnerable to damage from UV light. How Tumors Form Normally, cells in the body grow, divide, and produce more cells as needed. But sometimes the process goes wrong -- cells become abnormal and multiply in an uncontrolled way. These extra cells form a mass of tissue, called a growth or tumor. Tumors can be relatively harmless (benign) or cancerous (malignant). A malignant tumor can spread, damage healthy tissue, and make a person ill. 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. Basal Cell and Squamous Cell Carcinoma Basal cell and squamous cell carcinomas are very common in both older and younger people and are rarely life-threatening. Melanoma is a less common, yet more serious, type of skin cancer. Basal cell and squamous cell carcinomas are often called non-melanoma skin cancers or keratinocyte cancers. Melanoma Melanoma results from the uncontrolled growth of melanocytes and can occur anywhere on the body where melanocytes are located, including the skin, eyes, mouth and gastrointestinal tract. Men tend to develop melanoma more often on the trunk (the area from the shoulders to the hips) or the head and neck. Women more often develop melanoma on the extremities (arms and legs). Melanoma is found most often in adults, but can occur in children and teenagers. Melanoma is the most serious and most aggressive (fastest growing) form of skin cancer. An estimated 76,690 new cases of melanoma will be diagnosed in 2013. Because it is difficult to adequately treat melanoma after it has spread, the disease is expected to claim the lives of approximately 9,480 Americans in 2013. Can Skin Cancer Be Treated? Most basal cell and squamous cell skin cancers can be cured if found and treated early. Melanoma can often be treated effectively if caught in time.