Skin tumors are supposed to develop through accumulations of genetic and/or epigenetic events in normal cells of the skin. Among them, we focus on common skin tumors, including benign, seborrheic keratosis, and malignant, squamous cell carcinoma and melanoma. Many important molecules have been detected on the molecular tumorigenesis of each of them to date, and some drugs targeted for their molecules have been already developed. We review updates on the molecular tumorigenesis of these tumors with our current works.