Thymoma is a rare epithelial tumor of the thymus, but the most common malignancy in the anterior mediastinum. A unique feature is its association with paraneoplastic syndromes, in particular myasthenia gravis. According to the WHO classification 6 histologic types of thymic epithelial tumors can be discriminated. Tumor stage according to MASAOKA is the most important prognostic factor. Non-invasive tumors (stage I) are usually completely resected and no further therapy is warranted. For incompletely resected tumors and locally advanced invasive thymomas (stage Ill-IV) postoperative radiotherapy with 50-60 Gy is advisable. Chemotherapy, preferably with Cisplatinum, is indicated with inoperable thymomas or metastatic disease. In general thymomas have a fair prognosis even in advanced stage. Long term follow-up is mandatory up to 10 years.