A review and analysis of 150 cases of organoid nevus is offered. Most of the nevi are situated on the scalp and correspond to the type called nevus sebaceus of Jadassohn. A short historical review of Jadassohn's original work is used to clarify the definition of "nevus." By old usage this term is applied to lesions containing nevus cells. Jadassohn's definition of organoid nevi includes stable localized malformation of the skin through excess or deficiency of one or more of the normal mature constituents, such as hair, glands, epidermis, or connective tissue, and it excludes adenomas and other less mature tumors. Analysis of our 150 cases shows that the so-called nevus sebaceus also usually involves other skin constituents, and sometimes most of them. Its life history includes three stages. An early stage in infancy and childhood often is characterized by underdevelopment of hairs and sebaceous glands. A second phase usually begins at puberty and leads to massive development of sebaceous glands, papillomatous epidermal hyperplasia, and maturation of apocrine glands, which are present in a considerable number of cases. The third stage is due to the complicating development of benign and malignant nevoid tumors in the original nevus. Among the 52 tumors found in our series, basal cell epitheliomas and hidradenomas of various types were in the majority. The practical consequence of this analysis for prognostication and treatment of organoid nevi are outlined.