Digestive System Tumours is the first volume in the 6th edition of the WHO Classification of Tumours series, also known as the WHO Blue Books. This edition bridges evidence gaps identified in the previous edition, as well as providing new images, up-to-date information, and fully revised ICD-O-4 codes. The WHO Classification of Tumours Editorial Board is now guided on specific topics by seven newly formed subcommittees, and diagnostic criteria have been revised to balance scientific advances with global relevance, thanks to valuable input from representatives from resource-challenged settings. The WHO Blue Books are regarded as the gold standard for the diagnosis of tumors and comprise a unique synthesis of histopathological diagnosis with digital and molecular pathology. These reference books provide indispensable international standards for medical specialists involved in the care of patients with cancer, and for collecting epidemiological data, underpinning not only individual patient treatment but also research into all aspects of cancer causation, prevention, therapy, and education. Access the entire series online: WHO Classification of Tumours Online now hosts whole slide images of almost all tumours in the series, as well as additional figures, full reference lists, clickable citations, and more. Access to the full series is available at https: //tumourclassification.iarc.who.int.