Based on the original proposals of Müller-Hermelink [3,8] and the study of 95 tumor specimens from the files of our institute we have established a new concept for the classification of thymic epithelial tumors. Thymomas are related to the structural components of normal thymus and divided in medullary, mixed, predominantly cortical and cortical types. In addition a well-differentiated thymic carcinoma with partial loss of organotypic differentiation is characterized and distinguished from other carcinoma types with total lack of specific thymic features. Prognostic evaluation showed, that medullary and mixed thymomas are always benign tumors, whereas predominantly cortical thymomas, cortical thymomas and well-differentiated thymic carcinomas are low-grade malignant tumors with increasing invasiveness and even metastatic capacity. Moreover the proliferation rate of neoplastic epithelial cells in vitro, which was studied in 12 cases, correlated to the different tumor types and their growth behaviour in vivo.