The keratoacanthoma (KA) is a relatively common tumor which most often occurs on the sun-exposed areas of light skinned individuals of middle age and older. It may be viewed as an aborted squamous cell carcinoma that only in rare instances evolves into a progressively growing squamous cell carcinoma. As such, the recognition of the true nature and the proper management of this tumor is of considerable practical importance. The rapid evolution of a KA may be divided into a proliferative stage, a fully developed tumor, and an involuting stage. Within weeks it develops from a rapidly growing, firm, smooth nodule into a mature bud or dome-shaped tumor with a central keratotic core that finally degenerates into an involuting keratinous mass. Although the KA usually appears as a solitary lesion, multiple tumors may be found, as may a number of morphologic and syndromic variants, including the appearance of the KA in the Torre syndrome. The KA is likely to be derived from cells of the hair follicle. Its etiology is unclear, although ultraviolet light, viruses, oncogenic chemicals, and epidermal growth factor have been considered. The histologic features of the KA are often very similar to those of a cutaneous squamous cell carcinoma; however, the tumor architecture usually provides a basis for their distinction.