A universal and consistent stage classification system, which describes the anatomic extent of a cancer, provides a foundation for communication and collaboration. Thymic epithelial malignancies have seen little progress, in part because of the lack of an official system. The International Association for the Study of Lung Cancer and the International Thymic Malignancies Interest Group assembled a large retrospective database, a multispecialty international committee and carried out extensive analysis to develop proposals for the 8th edition of the stage classification manuals. This tumor, node, metastasis (TNM)-based system is applicable to all types of thymic epithelial malignancies. This article summarizes the proposed definitions of the T, N, and M components and describes how these are combined into stage groups. This represents a major step forward for thymic malignancies.