The World Health Organization (WHO) Classification of Tumours of Haematopoietic and Lymphoid Tissues is a comprehensive reference for the classification and diagnosis of hematological malignancies. This 4th edition, revised and updated, provides a detailed framework for the classification of myeloid and lymphoid neoplasms based on clinical, morphological, immunophenotypic, and genetic features. The classification aims to define distinct, clinically relevant entities that are mutually exclusive and collectively exhaustive, ensuring that diseases are accurately diagnosed and treated. The classification includes a wide range of neoplasms, such as myeloproliferative neoplasms, myeloid/lymphoid neoplasms with eosinophilia, myelodysplastic/myeloproliferative neoplasms, and myelodysplastic syndromes. It also covers various lymphoid neoplasms, including B-cell and T-cell neoplasms, as well as Hodgkin and non-Hodgkin lymphomas. The classification emphasizes the importance of integrating genetic and molecular data with clinical and morphological findings to ensure accurate diagnosis and classification. The WHO classification is based on the principles of the Revised European-American classification of lymphoid neoplasms (REAL), which emphasizes the importance of defining 'real' diseases that can be recognized by pathologists using available techniques. The classification incorporates the latest molecular and genetic insights, including the identification of specific genetic abnormalities that are key to the diagnosis and classification of certain neoplasms. It also includes new provisional entities and revised definitions for existing ones, reflecting the evolving understanding of hematological malignancies. The classification is designed to be a flexible framework that can incorporate new data and insights as they become available. It is intended to be used in clinical practice for therapeutic decision-making and to provide a basis for future research. The WHO classification is a collaborative effort involving experts from around the world, and it is continuously updated to reflect the latest scientific and clinical advancements in the field of hematological malignancies. The classification is essential for ensuring that patients receive accurate diagnoses and appropriate treatments based on the latest evidence and guidelines.The World Health Organization (WHO) Classification of Tumours of Haematopoietic and Lymphoid Tissues is a comprehensive reference for the classification and diagnosis of hematological malignancies. This 4th edition, revised and updated, provides a detailed framework for the classification of myeloid and lymphoid neoplasms based on clinical, morphological, immunophenotypic, and genetic features. The classification aims to define distinct, clinically relevant entities that are mutually exclusive and collectively exhaustive, ensuring that diseases are accurately diagnosed and treated. The classification includes a wide range of neoplasms, such as myeloproliferative neoplasms, myeloid/lymphoid neoplasms with eosinophilia, myelodysplastic/myeloproliferative neoplasms, and myelodysplastic syndromes. It also covers various lymphoid neoplasms, including B-cell and T-cell neoplasms, as well as Hodgkin and non-Hodgkin lymphomas. The classification emphasizes the importance of integrating genetic and molecular data with clinical and morphological findings to ensure accurate diagnosis and classification. The WHO classification is based on the principles of the Revised European-American classification of lymphoid neoplasms (REAL), which emphasizes the importance of defining 'real' diseases that can be recognized by pathologists using available techniques. The classification incorporates the latest molecular and genetic insights, including the identification of specific genetic abnormalities that are key to the diagnosis and classification of certain neoplasms. It also includes new provisional entities and revised definitions for existing ones, reflecting the evolving understanding of hematological malignancies. The classification is designed to be a flexible framework that can incorporate new data and insights as they become available. It is intended to be used in clinical practice for therapeutic decision-making and to provide a basis for future research. The WHO classification is a collaborative effort involving experts from around the world, and it is continuously updated to reflect the latest scientific and clinical advancements in the field of hematological malignancies. The classification is essential for ensuring that patients receive accurate diagnoses and appropriate treatments based on the latest evidence and guidelines.