Skin cancers are cancers that arise from the skin. They are due to the development of abnormal cells that have the ability to invade or spread to other parts of the body.[10] There are three main types of skin cancers: basal-cell skin cancer (BCC), squamous-cell skin cancer (SCC) and melanoma.[1] The first two, along with a number of less common skin cancers, are known as nonmelanoma skin cancer (NMSC).[5][11] Basal-cell cancer grows slowly and can damage the tissue around it but is unlikely to spread to distant areas or result in death.[5] It often appears as a painless raised area of skin that may be shiny with small blood vessels running over it or may present as a raised area with an ulcer.[1] Squamous-cell skin cancer is more likely to spread.[5] It usually presents as a hard lump with a scaly top but may also form an ulcer.[2] Melanomas are the most aggressive. Signs include a mole that has changed in size, shape, color, has irregular edges, has more than one color, is itchy or bleeds.[3]

More than 90% of cases are caused by exposure to ultraviolet radiation from the Sun.[4] This exposure increases the risk of all three main types of skin cancer.[4] Exposure has increased, partly due to a thinner ozone layer.[5][12] Tanning beds are another common source of ultraviolet radiation.[4] For melanomas and basal-cell cancers, exposure during childhood is particularly harmful.[6] For squamous-cell skin cancers, total exposure, irrespective of when it occurs, is more important.[4] Between 20% and 30% of melanomas develop from moles.[6] People with lighter skin are at higher risk[1][13] as are those with poor immune function such as from medications or HIV/AIDS.[5][14] Diagnosis is by biopsy.[3]