Mammalian ontogenesis and postnatal histogenesis involves the dynamic and appropriate interaction of two growth related phenomena: progression and regression. The thymus gland is the organ of the mammalian body that exhibits the most profound involution during normal postnatal histogenesis. Involution of the thymus can be compared to similarly regressive processes during the ontogeny of holometabolic insects, as well as to the spontaneous regression of neoplasms. It can be expected that in the future a better understanding of neoplastic regression will result from the comparison of ontogenetic processes from taxonomically far-removed regressive processes, and the evaluation of various factors that promote progression and regression. Furthermore the thymus, as the key organ of the cellular arm of the mammalian immune system, is intimately involved in the determination of the eventual outcome of malignant neoplastic disease. Primary anterior mediastinal tumors comprise a diverse group of neoplasms accounting for 50% of all mediastinal tumor lesions. Thymic carcinomas and thymomas are epithelial neoplasms which can be divided according to clinical behavior into various biological subtypes. A review of the nature of thymic neoplasms, a description of physiologic and accidental involution of the thymus gland, and a discussion of various paraneoplastic disorders, including myesthenia gravis, associated with thymic neoplasms are the aims of this article.