Since the publication of the AFIP Third Series fascicle in 1997, the World Health Organization (WHO) has printed two major volumes concerning tumors of the thymus. Drs. Mukai and Shimosato, who are not only two of the authors of Tumors of the Mediastinum but also co-authors of the Third Series fascicle, were collaborators on the WHO volumes. In the last 10 years, little progress has been made in the study of tumors of the thymus, although a few new subtypes have been reported. The most significant change was the subtyping by the WHO of tumors of the thymus that do not cause constitutional changes. The present Fascicle has adopted the WHO system of classification. For pathologists who are not as familiar with the WHO system, the authors extensively explain and illustrate both the morphologic and histogenetic classification systems that are its bases. Due to the changes in nomenclature, the chapter on thymic epithelial tumors has been considerably revised. The main diagnostic terms are presented along with the previous conventional morphologic and histogenetic terms, for the readers’ sake. Newly recognized tumor entities, new cytologic and genetic findings, and recent references have been added. Tables have been revised to reflect new data on survival, staging, and classification. This volume will be an authoritative reference on mediastinal tumors for years to come.