The thymus remains one of the least understood organs in the body. It has gone from the villain to the hero. For many years, it was blamed for what is now recognized as the sudden infant death syndrome; now, its role of immunosurveillance is being recognized. In spite of the fact that there are only two predominant cell types within the thymus, there are nearly 15 histologically different neoplasms of the thymus. These, in turn, are associated with more than 20 parathymic syndromes that affect approximately 40% of patients with thymoma. The three most common of these syndromes associated with thymic disorders are myasthenia gravis (MG), pure red cell aplasia (PRCA), and hypogammaglobulinemia. Thymomas are found in 15% of patients with MG, 50% of those with PRCA, and 10% of those with adult-onset hypogammaglobulinemia. Of all thymomas, 35% are malignant, that is, invasive or metastatic.