Since the publication of the third edition in 1997, the World Health Organization (WHO) has printed two major volumes concerning tumours of the thymus, and the authors of this volume collaborated on these books. In the last 10 years, little progress has been made in the study of tumours of the thymus, although a few new subtypes have been reported. The most significant change was the subtyping by the WHO of tumours of the thymus that do not cause constitutional changes. The present volume has adopted the WHO system of classification. For pathologists who are not as familiar with the WHO system, the authors clearly 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, although the main diagnostic terms are presented along with the previous conventional morphologic and histogenetic terms. Newly-recognised tumour entities, new cytologic and genetic findings, and recent references have been added, and the tables have been revised to reflect new data on survival, staging, and classification. This volume will be an authoritative reference on mediastinal tumours for years to come.