A wide variety of skin signs have been associated with internal malignancy. Cutaneous manifestations may develop before an internal neoplasm is recognized, and the skin findings can aid in diagnosis. In some cases the skin is infiltrated by malignant cells that represent metastatic or local spread from an internal malignancy. In other cases, the skin lesions are related to the underlying presence of malignancy, but they do not contain malignant cells. This issue of Dermatologic Clinics contains information on topics such as malignancy and cancer treatment-related hair and nail changes, tumor invasion of the skin, radiation therapy toxicity to the skin, and epidermal manifestations of internal malignancy.