The 2019 WHO classification of tumours of the digestive system reflects significant advancements in understanding these tumours, with some defined by molecular phenotype rather than histological characteristics. Histopathological classification remains the gold standard for diagnosis. The classification is designed for global use, even in settings with limited resources. Since the 2010 edition, there have been important developments in the understanding of tumour aetiology and pathogenesis. The new classification includes molecular markers relevant to diagnosis, biological behaviour, outcome, and treatment, rather than molecular pathogenesis. Molecular pathology is expanding, with some tumours requiring molecular analysis for accurate diagnosis. Notable changes include the classification of neuroendocrine neoplasms (NENs) into well-differentiated NETs and poorly differentiated NECs, based on molecular differences. The classification also includes a two-tiered system for precursor lesions, replacing the previous three-tier system. The terminology for precursor lesions has been standardised, with 'dysplasia' and 'intra-epithelial neoplasia' still acceptable in certain locations. The classification of liver tumours has been refined, with new subtypes defined by molecular and clinical features. The classification of pancreatic tumours has been updated, with precursor lesions classified into two tiers. Mixed tumours remain uncertain, with ongoing research needed. A new chapter on genetic tumour syndromes has been added, detailing major syndromes and their genetic basis. The format of the classification has been updated for clarity and to include larger illustrations. The classification is a collaborative effort by the WHO Classification of Tumours Editorial Board, with authors disclosing potential conflicts of interest. The classification aims to improve diagnosis, treatment, and research in the field of digestive system tumours.The 2019 WHO classification of tumours of the digestive system reflects significant advancements in understanding these tumours, with some defined by molecular phenotype rather than histological characteristics. Histopathological classification remains the gold standard for diagnosis. The classification is designed for global use, even in settings with limited resources. Since the 2010 edition, there have been important developments in the understanding of tumour aetiology and pathogenesis. The new classification includes molecular markers relevant to diagnosis, biological behaviour, outcome, and treatment, rather than molecular pathogenesis. Molecular pathology is expanding, with some tumours requiring molecular analysis for accurate diagnosis. Notable changes include the classification of neuroendocrine neoplasms (NENs) into well-differentiated NETs and poorly differentiated NECs, based on molecular differences. The classification also includes a two-tiered system for precursor lesions, replacing the previous three-tier system. The terminology for precursor lesions has been standardised, with 'dysplasia' and 'intra-epithelial neoplasia' still acceptable in certain locations. The classification of liver tumours has been refined, with new subtypes defined by molecular and clinical features. The classification of pancreatic tumours has been updated, with precursor lesions classified into two tiers. Mixed tumours remain uncertain, with ongoing research needed. A new chapter on genetic tumour syndromes has been added, detailing major syndromes and their genetic basis. The format of the classification has been updated for clarity and to include larger illustrations. The classification is a collaborative effort by the WHO Classification of Tumours Editorial Board, with authors disclosing potential conflicts of interest. The classification aims to improve diagnosis, treatment, and research in the field of digestive system tumours.